An interval defines a block of time that is tracked. The syntax for specifying an interval is flexible, and may be one of:
[from] <date> [from] <date> to/- <date> [from] <date> for <duration> <duration> before/after <date> <duration> ago [for] <duration>
from 9:00 from 9am - 11am from 9:00:00 to 11:00 from 9:00 for 2h 2h after 9am 2h before 11:00 2h ago for 2h
An interval is said to be 'closed' if there is both a start and end, and 'open' if there is only a start.