Using between, or adding a random number of X time to a date

[electronicalias]

I have multiple timestamps in the streamed record I'm trying to reproduce. Each one has a start and end date. I'm trying to randomise the end date by adding a random number of milliseconds to the start date.

I get the start date from {{date.utc}} and I can see there is a function for date.between() - however it's not clear how you would use this. I'm stuck on how to get the current date into the function:

{{date.between(<current_date>,<num_of_milliseconds>)}} doesn't work and neither have any of my other guesses. Any help on how to format this would be appreciated.