Dates are YYYY-MM-DD and timestamps are UTC, ending in Z. A match's played_on is the UTC date it was played.
Fixture times
Fixtures carry starts_at, the start time in UTC, ending in Z like every other timestamp. It is null while the order of play gives no time yet, rather than a guess.
How fresh an answer is
Every response carries meta.data_as_of, a UTC timestamp of how fresh the archive was when it answered. Live scores also carry freshness.captured_at; see Live and freshness.