Can we get the timestamp of opening the sail form for the first time?

You can just make a simple db which stores date and time when form is opened, the very first entry will obviously be the date and time when the form was opened at first, for getting that simply use function now()

Let me know if you want me to elaborate it

I would suggest wrapping the form in a load, with a variable that is populated with now() as suggested above. However, it depends what you're trying to achieve as the task could be opened, and subsequently reassigned to a different user, in which case you may wish to look at the task metrics: forum.appian.com/.../Process_and_Report_Data.html