I have looked through all the documentation related to string parsing, but could not find a way to solve the problem. If you use “save $DATE:%s$.fit” in the example above, it will create “save 2023-03-28T20:09:42.413.fit” from the FITS header. The problem is that there are “:” in the time string part, and I want to replace it with “_”, but I could not find a way to do this in the documentation.