The time of signing in UTC format.
When you call the Sign function, the UTC time is automatically
Note that (while reading or validating a PDF file,) this
property can only be relied on if the certificate is valid. You can
check whether the certificate is valid using the Validate method.
You will not need to change the value of this property unless
you are writing low-level signature manipulation code.
If the format of the date in the PDF is incorrect, then querying
the value of this property may result in an error being raised.