Serializing as ISO8601 makes the hash brittle, because the database may
change the timezone silently and/or lose precision in the microseconds.
As we do not need precise timestamp, using an integer is good enough,
and is consistant with the git format.
The manifest also does not need to contain a timezone, as it only
represents the timezone of the system that fetched this metadata,
which is useless data.
Depends on D4970.