This one is easy because Thunkable is using seconds since 1970 and what I do is connect a Google sheet as my data source and this sheet has two columns
Unix-time and normal-time
In the column normal time I have a formula
=A2/86400+DATE(1970,1,1)+TIME(5,30,0)
Then in Thunkable I update cell A2 and read cell B2 which has my answer.
I use it to compute number of dates between two dates and other date functions.