Fix potential timedata overflow

Summary:
The add before division can potentially cause an overflow.
If we divide before adding, no overflow can occur.

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6025