You need to work with the Date and Time functions that are available from a VB point of view. These are DateDiff, DateAdd and TimeValue.

I created the following code to test:
dim dtVar1
dim dtVar2
Integer iSeconds
dim tsDiff

let dtVar1=Now()
let dtVar2=DateSerial(2018,3,25)
let iSeconds=DateDiff('S',dtVar1,dtVar2)
let tsDiff=TimeValue(DateAdd('S',iSeconds,DateSerial(2018,1,1)))
The dtVar1 and dtVar2 can be replaced by references to a database column. The values of DateSerial in the last function are just to have a start value, to me they can be anything (as long as it is a valid date).