simple maths and chronology of events will give you a clearer picture:-

there are 2 starting points:-

for freehold properties then from the date of your sale and purchase agreement and for leasehold properties, the date of the state consent.

for example:-

date of sale and purchase agreement 15/01/2017 then your date of ownership starts on 15/01/2017.

in order to run from that real property gains tax, you need to dispose of that property on or after 16/01/2022.