Transform timestamp into date

I’m trying to develop a connector for Unleashed. The latest hurdle I am trying to overcome is that the dates are given as timestamps in the following format “/Date(1600300800000)/”.

What is the best way of going about this? The idea of a custom transformation is scary, and I’m not even sure how I would go about extracting the timestamp from this string and converting it in SQL.

Any help would be greatly appreciated!

Thank you!