Trying to detect recurring events at certain intervals, e.g. daily, weeekly, monthly, annually
Given an event type and a sequence of times at which it has occurred, there are several ways to check whether it seems to recur periodically.

One simple approach is to look at the interarrival times (the time between each occurrence and the previous occurrence), and histogram those; if it is periodic, you'll see a sharp peak in the histogram.

Another approach is to use autocorrelation to detect periodicity.  See, e.g., https://en.wikipedia.org/wiki/Autocorrelation.  The Fourier transform can be useful here.