def timestr_mod24(timestr: str) -> int:
    """
    Given a GTFS HH:MM:SS time string, return a timestring in the same
    format but with the hours taken modulo 24.
    """
    try:
        hours, mins, secs = [int(x) for x in timestr.split(":")]
        hours %= 24
        result = f"{hours:02d}:{mins:02d}:{secs:02d}"
    except:
        result = None
    return result
Given a GTFS HH:MM:SS time string, return a timestring in the same
    format but with the hours taken modulo 24.