Add a quick test that ensures `ganttDb` correctly adds `1d` (1 day),
even on days with 25 hours.
This test only runs if the test PC has the TZ='America/Los_Angeles'
set (California has daylight savings).
I've added a test to the GitHub Actions `test.yml` action too for this.
It should only add about 5 seconds to each test, so it isn't too bad.