The c# statement:
is taking the String representation of the Timestamp field and using a ToString overload to call String.Format("yyyy-MM-dd hh:mm:ss")
That is, the cited code is not keeping a reference to the timestamp, it is just formatting the value of the property as a string.
You can get the timestamp from any packet instance like so:
(apologies if syntax is wrong, I don't do much VB)
Dim dateValue As
Date = myPacket.Timestamp
and then do whatever you need to do with the timestamp value.
You'll find a lot of information about DateTime formatting options here: