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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 openTypeHeadCreatedFallback(info):
 """ Fallback to the environment variable SOURCE_DATE_EPOCH if set, otherwise now. """

    if "SOURCE_DATE_EPOCH" in os.environ:
        t = datetime.utcfromtimestamp(int(os.environ["SOURCE_DATE_EPOCH"]))
        return t.strftime(_date_format)
    else:
        return dateStringForNow()