value with html_select_date

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{html_select_date prefix="s_" end_year=+1 month_format=%m day_format=%02d field_order="YMD" field_separator="-"}
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i had my SELECT tag written as above.
i've stated that my day_format should be %02d , but why the HTML tags appear like the following?

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<select name="s_Day">
<option label="01" value="1">01</option>
<option label="02" value="2">02</option>
<option label="03" value="3">03</option>
・・・
</select>
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i just cant get the value to be
value="01"