Dates

The strict date grammar: months, relative days, durations, calendar semantics, output rules and errors.

Main branch reference after R89 · released versions may differ docs/SYNTAX_REFERENCE.md @ 1ebc752

Dates are a strict, bounded part of the language: May 5 + 3 weeks works, May 5 + 43 is an error, and a lone month word is just prose. Arithmetic is component-based on the local Gregorian calendar — not seconds-since-epoch — so daylight-saving transitions cannot shift a day.

This page is the complete date grammar: the accepted month names and abbreviations, today/tomorrow/yesterday, the duration words, the output year rule, and every documented limit and error case.

Grammar

<date> [ <sign> <integer> <duration> ]
<date>     := today | tomorrow | yesterday
             | <month> <day>
             | <month> <day>, <year>
             | <day> <month> [<year>]
<duration> := day(s) | week(s) | month(s) | year(s)

All words are English and case-insensitive. The sign is + or − (the ASCII - works too).

Months

#	Abbreviations	Full name
1	`jan`	`january`
2	`feb`	`february`
3	`mar`	`march`
4	`apr`	`april`
5	`may`	`may`
6	`jun`	`june`
7	`jul`	`july`
8	`aug`	`august`
9	`sep`, `sept`	`september`
10	`oct`	`october`
11	`nov`	`november`
12	`dec`	`december`

Relative dates

today, tomorrow and yesterday resolve against the local calendar. They can be combined with a duration: tomorrow + 1 month, yesterday - 2 weeks.

Durations

The accepted duration words are day, days, week, weeks, month, months, year, years. Durations are component-based: one month from January 31 is not “31 days later” — the calendar components are advanced, and the local-noon anchor keeps the day stable across daylight-saving changes.

Calendar semantics

Gregorian calendar.
Component arithmetic on Calendar — not 86400-second multipliers.
Anchored to local noon so that DST transitions never shift a day.

Output year

The result year is shown when the input specified a year, or when the result moves outside the input year.

Limits and errors

Periods up to 1,000,000; anything larger is a hidden error.
Feb 30 is date-shaped but not a valid date — a hidden error.
May 5 + 43 (a number without a duration word) is an error — never “reduced” to plain numbers.
May 5 + 43 days extra — a tail after a valid date — is an error.
A lone month word without a day (may) is not a date; it stays ordinary prose.

Examples

today                 → today (per the local calendar)
tomorrow + 1 month
yesterday - 2 weeks
May 5                 → May 5 (current year)
May 5, 2026 + 30 days
5 May 2026
dec 31

Sheet language — how date-shaped lines claim their text strictly.
Units & quantities — durations as catalog time units (1 day, 2 weeks) when used in quantity expressions.