Claude writes me a working Python script, then one small change later something unrelated is broken and I cannot tell what moved.

I am in claims operations, not a developer. The script reads our claims spreadsheet, finds anything with no update in fourteen days, and writes those rows to a second sheet so we can chase them. It worked on Tuesday. I asked for one change, to skip claims already marked closed, and after that it started counting weekends inside the fourteen days, which it was not doing before. I never asked it to touch the date logic. I am also no longer sure which file on my desktop is the version that worked. Is there a way of working that does not end with me starting again every two days?