BUG: Period[us] start_time off by 1 nanosecond
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Updated to catch overflow correctly. ping on green updated with a fix to missing import, which I expect will be causing most CI runs to fail until it gets patched ping gentle ping @WillAyd; this is a blocker