Compendium

Calendar / Scheduling / Recurrence DSLs Family Index

13 min read

Calendar / Scheduling / Recurrence DSLs Family Index

type: language-family-index family: calendar-scheduling languages_catalogued: 25 tags: [language-reference, family-index, calendar-scheduling, icalendar, vcard, rrule, cron, tzdata, jscalendar, jscontact, caldav, carddav, jmap]

Calendar / Scheduling / Recurrence — Family Index

Family overview

Calendar and scheduling DSLs are among the most-deployed structured data formats on the planet, and almost no end user knows their names. Every meeting invitation in Outlook/Gmail/Apple Calendar is iCalendar bytes (RFC 5545, originally RFC 2445 in 1998); every contact card import/export is vCard (RFC 6350, originally RFC 2426 in 1996); every CI/CD job, Linux backup, Kubernetes scheduled task, and AWS Lambda timer is some flavor of cron. The combination of multi-decade stability, embedded sub-DSLs (RRULE, EXRULE, DTSTART/DTEND, TZID), and overlapping vendor extensions makes this one of the richest and most under-studied family of formats in the wider language landscape.

The defining tension across this family is interchange-format vs. server-side scheduling. iCalendar/vCard/JSCalendar/JSContact describe content — events, todos, journals, contacts — and propagate it across mail, sync, and synchronization protocols ( JMAP). The cron family describes recurring action — “run this command every 15 minutes” — without any concept of an event object or attendee. The two traditions occasionally meet (RRULE expresses “every weekday at 09:00”, which is roughly cron’s 0 9 * * 1-5), but they evolved in distinct ecosystems: IETF/CalConnect (calendar interchange) versus Unix V7 → systemd → Kubernetes (server scheduling). Inside the iCalendar world, RRULE is the embedded recurrence sub-DSL — about 30 keywords, 20+ properties, capable of expressing “the third Thursday of each month except December” in a single line.

The 2021–2024 window saw an overdue JSON modernization: JSCalendar (RFC 8984, July 2021) and JSContact (RFC 9553, May 2024) replace the line-folded BEGIN:/END: text format with idiomatic JSON, while preserving full semantic mapping back to iCalendar/vCard. JSCalendar 2.0 is in IETF draft (calext-jscalendarbis-15, intended publication 2026), and JMAP for Calendars (draft-ietf-jmap-calendars-26, expiring May 2026) is the JSON-RPC sync protocol replacing CalDAV. Adoption is slow because the ubiquity of iCalendar/CalDAV is itself the moat — Fastmail and Apple have led, but Google and Microsoft remain on the legacy stack.

Underneath everything sits the IANA Time Zone Database (tzdata, “Olson DB”), itself a small structured language describing every legislative time-zone rule on Earth via Rule/Zone lines. The current release as of writing is 2026b (April 22, 2026), following 2026a (March 2026, Moldova DST correction). Releases happen roughly 5–10 times per year, every time a government changes a DST boundary or renames a zone. POSIX TZ strings (EST5EDT,M3.2.0,M11.1.0) are the minimal embedded subset. Together, tzdata + POSIX TZ are the substrate every other format in this family relies on for correctness.

In our deep library

None catalogued at Tier 1/2 level — calendar/scheduling DSLs do not have standalone deep-library notes; they sit on top of host languages (C, Java, Python, JavaScript) that are catalogued separately.

Cross-reference:

  • api-description — CalDAV (RFC 4791) and CardDAV (RFC 6352) are WebDAV/HTTP API descriptions; JMAP is a JSON-RPC API; Google Calendar API and Microsoft Graph calendar are OpenAPI-described REST APIs.
  • notation-spec — RRULE, POSIX TZ strings, cron expressions, and IANA tzdata Rule/Zone syntax are all formal grammars that warrant cross-link to grammar/notation indexing.
  • build-devops — cron is the recurring-task layer in CI/CD (GitHub Actions, GitLab CI, Jenkins, Argo Workflows) and orchestration (Kubernetes CronJob, AWS EventBridge, systemd timers).
  • i18n-locale (if present) — CLDR carries calendar-system data (Hebrew, Islamic, Japanese, Buddhist, Persian, Coptic, Ethiopic) that is closely related but distinct from tzdata; CLDR concerns how to display a date, tzdata concerns which moment in UTC a wall-clock time refers to.
  • citation-formats — adjacent in spirit (long-lived structured-data formats with multiple competing modernizations).

Tier 3 family table — Calendar/contact data formats

FormatFirst appearedOriginTypeStatus (2026)URL
iCalendar (RFC 5545)1998 (RFC 2445) → 2009 (RFC 5545)IETF CALSIFY WG; Internet Mail Consortium → IETFLine-folded BEGIN:/END: text format; the universal calendar interchangeVery active — ubiquitous in every meeting invitation worldwide; minor errata accumulatinghttps://datatracker.ietf.org/doc/html/rfc5545
vCard 4.0 (RFC 6350)1995 (Versit) → 1998 RFC 2426 → 2011 RFC 6350Versit Consortium → IETFLine-folded contact-card formatActive — RFC 6350 (Aug 2011) remains current; extended by RFC 9554 (vCard for JSContact, 2024)https://datatracker.ietf.org/doc/html/rfc6350
jCard (RFC 7095)2014IETFJSON representation of vCard dataActive but partially superseded by JSContact (RFC 9553)https://datatracker.ietf.org/doc/html/rfc7095
xCard (RFC 6351)2011IETFXML representation of vCard dataActive, nichehttps://datatracker.ietf.org/doc/html/rfc6351
JSCalendar (RFC 8984)July 2021IETF CALEXT WG (Robert Stepanek, Neil Jenkins)JSON-native calendar data model; alternative/successor to iCalendarActive; JSCalendar 2.0 in draft (calext-jscalendarbis, intended 2026 pub)https://datatracker.ietf.org/doc/html/rfc8984
JSContact (RFC 9553)May 2024IETF CALEXT WGJSON-native contact data model; alternative/successor to vCardActive; companion RFC 9554 maps vCard ↔ JSContacthttps://datatracker.ietf.org/doc/html/rfc9553
VEVENT / VTODO / VJOURNAL1998 (RFC 2445)iCalendar sub-componentsEvent, task, and journal-entry sub-formats inside iCalendarActive, all three universally implementedhttps://datatracker.ietf.org/doc/html/rfc5545#section-3.6
VAVAILABILITY (RFC 7953)August 2016IETFCalendar-availability component for free-busy publicationActive but niche; CalDAV scheduling extensionhttps://datatracker.ietf.org/doc/html/rfc7953
VFREEBUSY1998 (RFC 2445)IETFBusy-time exchange component within iCalendar/iTIPActive, core to scheduling workflowshttps://datatracker.ietf.org/doc/html/rfc5545#section-3.6.4
VTIMEZONE / TZID1998 (RFC 2445)IETFTime-zone embedding within iCalendar; references IANA TZID stringsActive, the standard way calendars carry tz datahttps://datatracker.ietf.org/doc/html/rfc5545#section-3.6.5

Tier 3 family table — Recurrence / scheduling DSLs

FormatFirst appearedOriginTypeStatus (2026)URL
RRULE (iCalendar recurrence)1998 (RFC 2445) → 2009 (RFC 5545 §3.3.10)IETFEmbedded recurrence DSL: FREQ=WEEKLY;BYDAY=MO,WE,FR;UNTIL=...Active, the de facto recurrence language; ~30 keywords, 20+ propertieshttps://datatracker.ietf.org/doc/html/rfc5545#section-3.3.10
EXDATE / EXRULE1998 (RFC 2445)IETFRecurrence exception dates / rules within iCalendar (EXRULE deprecated in RFC 5545 in favour of EXDATE)Active (EXDATE); deprecated (EXRULE, removed from RFC 5545)https://datatracker.ietf.org/doc/html/rfc5545#section-3.8.5.1
Unix cron (5-field)1975 (V7 Unix, Brian Kernighan)AT&T Bell Labs / Vixie cron (1987 onwards)5-field schedule string m h dom mon dowVery active, the universal substrate; Vixie/ISC cron, cronie, dcron etc.https://man7.org/linux/man-pages/man5/crontab.5.html
Quartz cron (6-/7-field)~2001 (Quartz Java scheduler)OpenSymphony → Terracotta → Software AG; current Quartz 2.3.x and Quartz.NET 3.xCron with seconds (field 1) and optional year (field 7); adds ?, L, W, # operatorsActive, the Java/.NET enterprise scheduler standardhttps://www.quartz-scheduler.org/documentation/
Kubernetes CronJob schedule2017 (k8s 1.8 beta) → 2021 GA (1.21)Kubernetes (Google → CNCF)Standard 5-field cron interpreted by kube-controller-manager; supports @every and @daily aliasesVery activehttps://kubernetes.io/docs/concepts/workloads/controllers/cron-jobs/
GitHub Actions cron2019GitHub5-field cron, UTC-only; schedule: trigger; @yearly/@monthly etc. not supportedVery active, runs millions of jobs/dayhttps://docs.github.com/actions/using-workflows/events-that-trigger-workflows#schedule
AWS EventBridge cron2019 (CloudWatch Events → EventBridge rebrand)AWS6-field cron (adds year), uses ? wildcard for day-of-month/day-of-week mutual exclusionVery activehttps://docs.aws.amazon.com/scheduler/latest/UserGuide/schedule-types.html
AWS EventBridge rate(...)2016AWSSibling expression to cron: rate(5 minutes), rate(1 hour), rate(7 days)Activehttps://docs.aws.amazon.com/scheduler/latest/UserGuide/schedule-types.html
systemd OnCalendar2013 (systemd 212+)Lennart Poettering / Red Hat / freedesktop.orgNative systemd timer spec: OnCalendar=Mon..Fri *-*-* 09:00:00; superset of cronActive, the modern Linux replacement for cron in distros that adopted systemdhttps://www.freedesktop.org/software/systemd/man/systemd.time.html
at(1) time spec1979 (V7 Unix)AT&T Bell LabsOne-shot natural-language time spec: at noon tomorrow, at midnight + 1 weekActive but niche; lives alongside cronhttps://man7.org/linux/man-pages/man1/at.1.html
Holiday-rules DSLs (Workalendar / python-holidays)2014+Various OSS projectsCode-defined holiday rules (“first Monday of September”, “Easter + 50 days”) that emit iCalendarActive, dozens of country-specific rule moduleshttps://github.com/peopledoc/workalendar

Tier 3 family table — Sync / transport protocols

ProtocolFirst appearedOriginTypeStatus (2026)URL
CalDAV (RFC 4791)March 2007IETFWebDAV extension for calendar sync; HTTP REPORT/PROPFIND over iCalendar resourcesActive, the universal open standard; backbone of Apple Calendar, Thunderbird, Nextcloud, Radicale, Baikalhttps://datatracker.ietf.org/doc/html/rfc4791
CardDAV (RFC 6352)August 2011IETFWebDAV extension for vCard syncActive, sister to CalDAVhttps://datatracker.ietf.org/doc/html/rfc6352
iTIP (RFC 5546)December 2009 (obsoleted RFC 2446)IETFiCalendar Transport-Independent Interoperability Protocol; defines REQUEST/REPLY/REFRESH/CANCEL/COUNTER/DECLINECOUNTER/PUBLISH/ADD methodsActive, the meeting-invite workflowhttps://datatracker.ietf.org/doc/html/rfc5546
iMIP (RFC 6047)December 2010 (obsoleted RFC 2447)IETFiCalendar over email — the actual text/calendar; method=REQUEST MIME body in meeting-invite emailsVery active — every Outlook/Gmail meeting invite in your inbox is iMIPhttps://datatracker.ietf.org/doc/html/rfc6047
JMAP for CalendarsInternet-Draft (draft-ietf-jmap-calendars-26, expires May 2026)Fastmail / IETF JMAP WGJSON-RPC sync protocol over JSCalendar; intended CalDAV replacementIn progress, not yet RFC; Fastmail ships production supporthttps://datatracker.ietf.org/doc/draft-ietf-jmap-calendars/
JMAP for ContactsInternet-DraftFastmail / IETF JMAP WGJSON-RPC sync protocol over JSContact; intended CardDAV replacementIn progresshttps://jmap.io/spec.html
Microsoft Exchange Web Services (EWS)2007 (Exchange 2007)MicrosoftXML SOAP-based calendar/mail/contact API; proprietaryLegacy — Microsoft has deprecated EWS in favour of Microsoft Graph for cloud Exchange Online (announced retirement 2026)https://learn.microsoft.com/exchange/client-developer/exchange-web-services/exchange-web-services-reference-for-exchange
Microsoft Graph (calendar/contact)2015MicrosoftREST + JSON event/contact resources; OAuth 2.0Very active, the strategic API for Microsoft 365https://learn.microsoft.com/graph/api/resources/event
Google Calendar API2011 (v3)GoogleREST + JSON event resourcesVery activehttps://developers.google.com/calendar/api/v3/reference
Google People API (contacts)2017 (replaced Contacts API)GoogleREST + JSON person resourcesActivehttps://developers.google.com/people

Tier 3 family table — Time-zone databases & sub-DSLs

FormatFirst appearedOriginTypeStatus (2026)URL
IANA tzdata (Olson DB)1986 (Arthur David Olson, NIH) → IANA-stewarded since 2011A. D. Olson, P. Eggert, et al.Authoritative time-zone rules database; Rule, Zone, Link lines define DST/UTC-offset history per IANA TZIDVery active; current release 2026b (April 22, 2026, after 2026a’s Moldova DST correction); 5–10 releases/yearhttps://www.iana.org/time-zones
IANA TZID strings1986 (Olson DB)OlsonContinent/City naming convention: America/New_York, Europe/London, Asia/Tokyo, Pacific/AucklandActive, the canonical time-zone identifierhttps://www.iana.org/time-zones
POSIX TZ string1988 (POSIX.1)IEEE / The Open GroupMinimal embedded TZ format: EST5EDT,M3.2.0,M11.1.0Active, used by C tzset(), embedded systems, and as a fallback in IANA tzdata for far-future ruleshttps://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03
DTSTART / DTEND / DURATION1998 (RFC 2445)IETFTime-anchor sub-DSL within iCalendar; can be DATE-only, DATE-TIME, or DATE-TIME with TZIDActive, mandatory in every VEVENThttps://datatracker.ietf.org/doc/html/rfc5545#section-3.8.2

Total entries: 25.

Notable threads

  • iCalendar (RFC 5545) is one of the most-deployed text formats on Earth. Every meeting invitation in Outlook, Gmail, Apple Calendar, Thunderbird, Zoom, Teams, Notion, etc. is iCalendar bytes flying around — usually as an iMIP (RFC 6047) text/calendar; method=REQUEST MIME part inside an email, or as a CalDAV (RFC 4791) REPORT response. ~28 years of stability with one major revision (2009: RFC 2445 → RFC 5545) and one structural successor (2021: JSCalendar / RFC 8984). The format’s vertical-bar line-folding (75-octet limit, leading-space continuation) is famously irritating to parse, but the install base makes it impossible to deprecate.

  • The RRULE recurrence DSL is the hardest part of any calendar implementation. Roughly 30 keywords (FREQ, BYDAY, BYMONTHDAY, BYMONTH, BYSETPOS, BYWEEKNO, BYYEARDAY, INTERVAL, COUNT, UNTIL, WKST, …), and the interaction between BYxxx rules is non-obvious — RFC 5545 §3.3.10 includes a 6-page table of how BYxxx parts expand or limit one another. RRULE can express things like “the third Thursday of each month except December” (FREQ=MONTHLY;BYDAY=3TH;BYMONTH=1,2,3,4,5,6,7,8,9,10,11) or “the last weekday of every quarter” (FREQ=YEARLY;BYMONTH=3,6,9,12;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=-1). Most calendar UIs expose only a tiny subset; the full expressiveness is rarely advertised.

  • JSCalendar / JSContact (2021/2024) as the long-overdue JSON refresh. RFC 8984 (Stepanek/Jenkins, July 2021) and RFC 9553 (May 2024) replace the line-folded text format with idiomatic JSON, with full round-trip mapping back to iCalendar/vCard preserved (RFC 9554 provides the vCard ↔ JSContact map). Fastmail and Apple lead adoption; Google and Microsoft remain on iCalendar/CalDAV. The IETF CALEXT WG charter requires extensions to define both representations. JSCalendar 2.0 (calext-jscalendarbis-15) is in late draft as of 2026.

  • Cron’s universality from 1975 to 2026. The 5-field cron syntax originated in V7 Unix (Brian Kernighan) and Vixie cron (Paul Vixie, 1987), and has been carried unchanged through every Unix, Linux distro, BSD, container orchestrator, CI/CD platform, and FaaS runtime since. Quartz (Java, 2001) added seconds + year and Sunday=1 day-of-week (vs. Unix’s Sunday=0); AWS EventBridge (2016) adopted Quartz-style 6-field cron with ? wildcards; Kubernetes CronJob (2017+) stayed strict 5-field; GitHub Actions (2019) is 5-field UTC-only. Despite many proposed replacements, base 5-field cron remains the lowest common denominator.

  • systemd OnCalendar as cron’s modern successor on Linux. systemd timers (2013, systemd 212+) introduced an OnCalendar= syntax that is a superset of cron with cleaner semantics: OnCalendar=Mon..Fri *-*-* 09:00:00, full IANA-TZID awareness via Persistent=, randomized delays via RandomizedDelaySec=, and cleaner --monotonic boot-relative timers. Most major distros (Fedora, Arch, Debian, Ubuntu) ship systemd timers as the default replacement for cron; Alpine and OpenWrt remain cron-first. Cron is not going away in <10 years, but new code increasingly targets systemd.timer or container-orchestrator schedules instead.

  • IANA tzdata as a structured language. The Olson DB Rule and Zone lines are themselves a small DSL describing every legislative time-zone rule on Earth: Rule USA 1967 2006 - Apr lastSun 2:00 1:00 D declares that from 1967 to 2006, USA DST began on the last Sunday of April at 02:00 local with a 1-hour offset called “D”. Releases happen 5–10 times per year, every time a government changes a DST boundary, abolishes DST, or renames a zone. Current release: 2026b (April 22, 2026), after 2026a (March 2026) corrected Moldova’s upcoming clock transition. The data is consumed by glibc, musl, Go’s time/tzdata, Java tzupdater, .NET TimeZoneInfo, Python zoneinfo, Rust chrono-tz, and effectively every operating system on Earth.

  • POSIX TZ strings vs IANA tzdata: minimal vs rich. POSIX TZ (EST5EDT,M3.2.0,M11.1.0) is the minimal embedded format: standard-time abbreviation + offset, optional DST abbreviation + DST start/end rules. IANA tzdata is the rich, historically-accurate database. POSIX TZ cannot express historical changes (e.g. that the USA only adopted DST in 1966), and is used today mostly as an embedded fallback in IANA tzdata files for “future rules beyond the data” and in tiny embedded systems where the full tzdata blob is too large.

  • The “DST is a programming hazard” tradition. Nearly every calendar bug ever filed traces to DST, time-zone math, or the spring-forward “non-existent local time” / fall-back “ambiguous local time” cases. Examples: Samoa’s 2011 jump across the date line (skipping December 30 entirely); Turkey’s 2016 abrupt DST abolition; Russia’s repeated DST policy reversals; Brazil’s 2019 DST abolition; Kazakhstan’s 2024 unification onto UTC+5. Calendar libraries (zoneinfo in Python, java.time in Java, Temporal in JS, chrono-tz in Rust) all emphasize “always store in UTC, only convert at the edges” — and that pattern only works because IANA tzdata exists.

  • vCard’s contact-card resilience. Versit Consortium (1995), IETF RFC 2426 (1998), RFC 6350 (2011, version 4.0), and now JSContact (RFC 9553, 2024) — vCard has survived 30+ years of contact-management upheaval (PalmPilot → BlackBerry → iPhone → Google Contacts → Microsoft 365 People → Fastmail). Every smartphone exports/imports vCard (.vcf) files; every business-card scanner emits vCard; every CRM-to-mailclient handoff goes through vCard or jCard. The format is close to iCalendar in syntax (line-folded BEGIN:VCARD/END:VCARD blocks) and shares the same vObject heritage.

  • Holiday calendars as a meta-language. Workalendar (Python), python-holidays, dateutil, .NET’s Nager.Date, JavaScript’s date-holidays, Ruby’s holidays gem — these libraries express country-specific holiday rules in code (“US Labor Day = first Monday of September”, “UK Spring Bank Holiday = last Monday of May”, “Japanese Coming-of-Age Day = second Monday of January”, “Easter + 50 days = Pentecost”) and emit iCalendar VEVENT streams. The rule definitions themselves are a small embedded DSL — typically a Python class with methods like get_first_monday_of(month) — and stay in sync with parliamentary changes (e.g. UK’s 2022 Queen’s Platinum Jubilee bank holiday, Japan’s 2019/2020 Emperor’s enthronement holidays).

Citations

Graph View