def parse_at_time_string(at_string):
    """Parses the '2:00s' string into '2:00' and 's'. If there is no suffix,
    returns a '' as the suffix.
    """
    modifier = at_string[-1:]
    if modifier.isdigit():
        modifier = ''
        at_time = at_string
    else:
        at_time = at_string[:-1]
    if modifier not in ['', 'w', 's', 'u', 'g', 'z']:
        raise Exception('Invalid AT modifier (%s)' % modifier)
    return (at_time, modifier)