It is a technical thing. In Adobe, they defined that this field is type "Date"
When defining it as type "Date" Adobe Reader shows you a drop down calendar. (this is shown by the tool, not defined by the IRCC)
Now, when you pick a date (say: 2017 - 10 - 05 )
The form field defined
in length to 7 character long, hence: YYYY-MM.
That is why Adobe form, cuts the
2017-10-01 to
2017-10.
Whoever put the form, tried to explain that the format of the field (in the end) is gonna be YYYY-MM.
Is it clear now?
Folks, don't elaborate too much! Though for an IT company, you would be a great testers!