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