Convert int to timestamp string.

def convert_to_timestamp(time):
    """Convert int to timestamp string."""
    if time == -1:
        return None
    time = int(time*1000)
    hour = time//1000//3600
    minute = (time//1000//60) % 60
    second = (time//1000) % 60
    return str(hour).zfill(2)+":"+str(minute).zfill(2)+":"+str(second).zfill(2)