Q re highly skilled worker visa expiry dates

I have calculated that I have enough points for the highly skilled worker visa (no sponsorship). I am going to apply next week as I collect all the documents, so jumping the gun a bit. But here is a question:
Is the two year expiry on the visa from the date of receiving it or from the date of entry into the UK? I.e. once I receive the visa (knock on wood) I will have to make the arrangements for so many things and obviously a few months will pass before I get there. Will I be losing these months as part of my allowed stay and only be able to work for, say 1.5 yrs before having to renew? or will the clock start ticking only when I enter the UK and then will I have the full 2 yrs to work before renewing?

Thanks!