My guess, it's the battery...
Manual has troubleshooting steps that point to battery.
1) Timer requires CR2 battery to work ... double check markings on your
battery. Programs will not function without battery. Battery can be low
without BATT warning on screen.
2) ST01C has 1 year limited warranty. Things wear out.
3) When reprogramming, do you Reset timer? Or just reprogram Time-Date. Last-ditch troubleshoot says to
Reset and Re-program timer.