from datetime import datetime

def uptime(start_datetime):
    """Returns the time elapsed since a given datetime.

    Given the 'start_datetime' returns a string with the elapsed time between
    now and start_datetime.

    If elapsed time > 24 then string format is:
        %d days, %H:%M:%S
    else:
        %H:%M:%S

    :param start_datetime: datetime
    :return: str
    """
    delta = datetime.now() - start_datetime
    days = delta.days
    hours, remainder = divmod(delta.seconds, 3600)
    mins, secs = divmod(remainder, 60)
    kwargs = dict(days=days, hours=hours, mins=mins, secs=secs)
    if days:
        return "{days} days, {hours:02d}:{mins:02d}:{secs:02d}".format(**kwargs)
    return "{hours:02d}:{mins:02d}:{secs:02d}".format(**kwargs)