Make adjusted time type safe

Summary:

This makes follow-ups easier to review. Also, it makes sense by itself.

This is a backport of core#25786

Depends on D15095

Needed for core#25717

Test Plan: cmake .. -GNinja -DBUILD_BITCOIN_CHAINSTATE=ON && ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15096