In a science fiction setting, what can explain an electronic calendar's date jumping around, backwards or forwards for days at a time, starting from a whole number like 01/01/2000, and bad things happen if it goes before that? The calendar is supposed to be in a sealed shelter after an apocalypse and, as far as I understand, the true date when the story happened is much later than year 2000.
I dreamed of reading a short story about an apocalypse shelter. It was presented in the form of one protagonist's diary apparently, and no direct interaction with other survivors were mentioned. He/she(I'm not sure) doesn't know what's the date that he entered the shelter, but at first the big calendar on the wall showed 01/01/2000 which was obviously wrong. He seemed to be trapped in a room alone with supplies and nothing to do. The weird thing is every morning when he wakes up, he sees that the date jumps around, backward or forward, for a few (up to 15 or so) days. If the date decreases an alarm rings that wakes him, and the number flashes red. Sometimes when the date goes before 01/01/2000, he thinks he can hear screams somewhere(he tried but could never talk with anyone), and the next day it would be back to 2000. If it jumps forward the number flashes green. Sometimes the date increments by 1 normally and doesn't flash in color, but it seems to never stay the same as yesterday. Over time he gets more and more afraid, not knowing what the date means, and thinks of wild possibilities to explain them. I don't remember what are his explanations - they don 't make a lot of sense, but are getting more and more horrible. Then I woke up, very scared, because in the dream I was immersed in and seeing the scenario as the story went on. I was relieved that it was not real, and regretful when I realized that I've never read any such story in real life. The ending (from impressions of blurbs of critics before the story) seems to be that everyone dies, but I'm curious what really went on and why that may happen.
What is a plausible science fiction explanation of this story scenario?