FillField with DateTime value

The file has an input box for dates like this:

2017-09-27_11-57-01.png (3.9 KB)

I filled in the field using FillField(fieldname, DateTime.Now.ToString());.

Validating the pdf shows the error, input not valid. How can I add valid DateTime to the field?

Please check the format of the form field in properties, and then modify the code. If this does not help, then kindly send us your source PDF document and code. We will investigate and share our findings with you.