def pad_time(timestring, pos='start'):
    """
    Returns a 12 digit string as time by padding any missing month, day, hour
    or minute values.

    If `pos` is `start`: Pad with year=0, month=1, day=1, hour=0, min=0
    If `pos` is `end`:   Pad with year=9999, month=12, day=31, 
                         hour=23, min=59

    Default is to pad with start position.
    """
    yr = int(timestring[:4])

    if pos == 'end':
        padder = "999912312359"
    else:
        # Default to start
        padder = "000001010000"

    if len(timestring) < 12:
        timestring = timestring + (padder[len(timestring):])

    return timestring