[ticket/17624] Fix datetime edge case tests failing in non-UTC timezones
The edge case test data used strtotime('yesterday'/'today'/'tomorrow')
which computes timestamps in the local timezone, then formatted them
with gmdate() which outputs in UTC. When the local timezone is ahead
of UTC, this shifts the date by one day, causing consistent failures.
Replace with time() arithmetic (time() - 86400, time(), time() + 86400)
which is timezone-agnostic and matches the pattern used by the other
test cases in the same data provider.
