import time


def elapsed_since(start):
    """Helper to calculate the elapsed time in ms, s, min or hrs.
    
    :param start (sec)      : starting time
    :return                 : elapsed time in ms, s, min or hrs
    """

    elapsed = time.time() - start
    if elapsed < 1:
        return str(round(elapsed * 1000, 2)) + 'ms'
    if elapsed < 60:
        return str(round(elapsed, 2)) + 's'
    if elapsed < 3600:
        return str(round(elapsed / 60, 2)) + 'min'
    else:
        return str(round(elapsed / 3600, 2)) + 'hrs'