[chore] Use chrono::UNIX_EPOCH instead of defining our own

[scovich]

A perfectly good constant already existed -- use it!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.54%. Comparing base (092ee67) to head (9326eda).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #371      +/-   ##
==========================================
- Coverage   76.55%   76.54%   -0.02%     
==========================================
  Files          45       45              
  Lines        9375     9374       -1     
  Branches     9375     9374       -1     
==========================================
- Hits         7177     7175       -2     
- Misses       1799     1801       +2     
+ Partials      399      398       -1     

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[zachschuermann]

lgtm!