A rigid cycle app breaks on this
Cycle apps are built on a loop that repeats. Perimenopause is the years that loop stops repeating — hormones swing rather than decline, a period arrives a week early and then not for sixty days, and the app quietly re-forecasts around a rhythm that has left.
Second Spring is built for the drift instead of against it. A gap is a reading, not an error. Nothing in it needs your cycle to behave in order to keep a usable record of the year you are actually having.
Built for the drift