Hi Thoness,
By default the date and time stamp are collected at form load but you can however change the rules of the form to collect upon checking the checkbox. If you notice on the Signature template part, if you check the properties for the DisplaySignedBy, SignedDate and SignedTime fields they have default values set on them. Meaning that those values would be set as soon as the form loads. What you can do is remove them from the default values and have those values set upon checking the checkbox instead. So if you click the validate checkbox and go to manage rules you should see a rule named Valid PIN. This rule has a condition set on it till only have its rules fire if the PIN entered is correct. Click that rule and add three more actions to replace the 3 default values that you removed:
1) Set a field Value > Field: SignedDate > Value: today()
2) Set a field Value > Field: SignedTime > substring-after(now(), "T")
3) Set a field Value > Field: DisplaySignedBy > concat("Signed by ", UserName, " on", " ", SignedDate, " ", "at", " ", SignedTime)
(for the third action you'll have to type it in and insert the UserName, SignedDate and SignedTime fields by Clicking Insert Field and selecting them from the field panel)
By doing this, only when you have entered the correct Username and PIN and have successfully checked the checkbox will the time/date stamp information be collected.
Hope that helps,
Mo