Add a note about possible failures with {build_date:%S}

[jwodder]

Closes #125.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.62%. Comparing base (9b6603e) to head (7d133ba).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #126   +/-   ##
=======================================
  Coverage   97.62%   97.62%           
=======================================
  Files          17       17           
  Lines        1263     1263           
  Branches      188      188           
=======================================
  Hits         1233     1233           
  Misses         15       15           
  Partials       15       15           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.