Revert "#25422 Breaking Change! Timestamps are now cached as UTC unixtime. Previously, timestamps were caches as local time zone based unixtime, to make it easy for clients to just pick up and display a value. However, this won't work if you travel with your laptop or when daylight savings time kicks in. This is a change in behaviour and is part of the versioning up to v2.0 of the whole framework. The cache generation is also versioned up to ensure that old caches are not interpreted against this new convention."

This reverts commit 53cfc44c18fdcb031222d8a73cdf2f307f601f8f.
[minor] Clarified some time zone code comments.