Reliability of CM15a with batteries during power outage

[SkipWX10]

I've never had batteries in my CM15a controller and have always updated the clock after an outage. Well I have to leave for about three weeks and am going to install batteries to test if they save timers and clock keeps time.

Does anyone know how long a new set of 4AAA batteries will last in an outage? Has it been reliable?

If not reliable, I'll disco and use the WM100 that I can at least connect to and update clock from the app.

Thanks!