Try resetting timer, and reprogramming.
The GE brand of electronic box timers is not dependable long-term product. The 30-40 amp contact ratings do not appear consistent with rest of timer quality. I think the amp load is overrated from what I have seen, compared with heavy duty timers like Intermatic GM40AVE series etc.
My local home depot has replaced the GE line of box timers with 100% reliable Intermatic T100 series mechanical timers.
15136 is not battery-operated timer, and there are no repair parts.
15136 timer is rated outdoors, with temperature range -40F to +130F, which sounds like hardened electronics.
But often these cheaper electronic devices with plastic gears and multi-voltage are not hardened for years of dependable service as found with commercial grade timers.
Nor are they protected from surge caused by local power outage or nearby lightning strike.
For most dependable timer, and cost effective, choose T100 series or T170 series with skip-a-day... repair parts available... timer will last decades