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I know timers and electrical devices.
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You posted under the Intermatic ST01C timer.
I assume this is not a new installation, so we know the wiring is correct, and your programming worked correctly in the past.
You have the timer set up for dusk to dawn.
The manual has troubleshoot section.
http://waterheatertimer.org/Intermatic-st01c.pdf
I have read the manual and tested&wired the timer and programmed the timer.
A low battery can cause this kind of problem, even if BATT is not flashing on screen.
The manual also says when encountering a problem, you can reset and then reprogram the timer.
The manuals says to check for loose wires, and defective bulb.
The timer does not work on circuits longer than 100'
The timer does not work if there are underground wires.
The symptoms you describe sound like loose wires or a light bulb.
I would check the wire nuts at the timer and at the outdoor fixture.
Change the outdoor bulbs.
As last resort, reprogram the timer.
These electronic devices go bad, and the ST01C has a 1 year warranty >>> combine the short warranty with the long troubleshooting section, I believe the product fails frequently.
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