Compute Leap Seconds
I think „UTC Timestamp“ is not the correct term since UTC is defined in terms of Leap Seconds.

What is a more usual term here is TAI time or GPS time, both use no leap seconds and UTC is defined in terms of TAI (GPS is basically TAI starting at a different offset, so it has 19s less leap time).

The difference of UTC time to TAI time cannot be calculated (unless you have both stamps, then you just need to subtract them). Instead, tTo get the official offset at any point in the past and some time in the Future you need to lookup the official tables, this is the authoritative source: http://hpiers.obspm.fr/eop-pc/index.php

I think some timezone Information files/libraries also have that information, but have not seen any support in commercial databases.