Every night my digital clock skips forward 30 minutes

I'm wondering what could cause my radio shack digital alarm clock (cat. # 12-1655) to advance 30 minutes every night, at some point in the night? I've caught it at 11:30pm displaying the right time still, and then by 2:30am it's reading 30 minutes fast but doesn't gain any more time until the next night. I haven't been able to catch it in the moment to see what time it makes the change and how fast it advances.

It holds time well the rest of the day, which is what really confuses me.

Edit for more info: -It's plugged in, not using battery power -I have tried the 2 outlets nearest and it does the same thing -I'm an electronics noob -It's my dad's alarm clock that I have now after he passed away in his sleep last summer and part of me embarrassingly wonders if the clock does something weird according to the time of his passing, even though I know it's not the case.

Thanks in advance!