In order to ensure the safety of the chain, the datestamp of the cast vote, namely the day and month (not the year) should be the date from which the ticker starts its countdown after HF25.

i.e. if a vote was cast on 01 August 2017 then the ticker will retroactively start its countdown on that one witness vote from 01 August 2020. (assuming HF25 is done before 01 August 2021)

So "retroactively" is the answer, but in no way all from the same date, as that could risk the chains existential security in 365 days from that common date it started.