Date 2017-07-00 recognized as 2017-06-00

Hi,
I am using 2017-07-00 date format to recognize month with no specific day.
But if I use 00 the month is automatically reduced by one month.
2017-07-00 --> 2017-06-00
2017-05-00 --> 2017-04-00
If the day is between 01 and 31 it works "normally" and 2017-07-00 --> 2017-07-00
Is this a feature or fault?
I am using 2017-07-00 date format to recognize month with no specific day.
But if I use 00 the month is automatically reduced by one month.
2017-07-00 --> 2017-06-00
2017-05-00 --> 2017-04-00
If the day is between 01 and 31 it works "normally" and 2017-07-00 --> 2017-07-00
Is this a feature or fault?
