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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def generate_timestamp(fmt=None):
 """ Generate a timestamp to mark when this file was last modified. :param str fmt: Special format instructions :return str: YYYY-MM-DD format, or specified format """

    if fmt:
        time = dt.datetime.now().strftime(fmt)
    else:
        time = dt.date.today()
    return str(time)