-- Datetime builtins: dtfmt (epoch -> R t t), dtparse (text -> R n t).
-- UTC only. Format strings follow strftime conventions.
-- Format a unix epoch as a date string. dtfmt returns a Result because the
-- epoch may be non-finite or out of range for chrono; the enclosing fn
-- returns R so the caller can pattern-match the result.
fmtday ts:n>R t t;dtfmt ts "%Y-%m-%d"
-- Parse a date string back to an epoch. dtparse returns a Result, so the
-- enclosing fn must also return R for `!` to short-circuit on failure.
parseday s:t>R n t;v=dtparse! s "%Y-%m-%d";~v
-- run: fmtday 0
-- out: 1970-01-01
-- run: fmtday 1735689600
-- out: 2025-01-01