The calendar vocabulary defines two data types and a set of operations on them:

timestamp

duration

Durations represent spans of time:
Using durations

Arithmetic on timestamps and durations:
Timestamp arithmetic

Getting the current timestamp:

now ( -- timestamp )

now-gmt ( -- timestamp )

now-utc ( -- timestamp )

Time zones:

>local-time ( timestamp -- timestamp' )

>gmt ( timestamp -- timestamp' )

>utc ( timestamp -- timestamp' )

convert-timezone ( timestamp duration -- timestamp )

Timestamps relative to each other:
Relative timestamps

Operations on units of time:
Year operations
Month names in English
Day names in English

Both timestamps and durations implement the Linear order protocol.

Solar position calculations:

sunrise ( timestamp latitude longitude -- new-timestamp )

sunset ( timestamp latitude longitude -- new-timestamp )

solar-noon ( timestamp longitude -- new-timestamp )

Metadata about the calendar:
Calendar facts