def iso8601_datetime(d):
if d == values.unset:<EOL><INDENT>return d<EOL><DEDENT>elif isinstance(d, datetime.datetime) or isinstance(d, datetime.date):<EOL><INDENT>return d.strftime('<STR_LIT>')<EOL><DEDENT>elif isinstance(d, str):<EOL><INDENT>return d<EOL><DEDENT>
Return a string representation of a date that the Twilio API understands
Format is YYYY-MM-DD. Returns None if d is not a string, datetime, or date