Can you extend your VISA early and not lose days?

We are on a UK 60 day tourist visa expiring on May 28th 2019. We plan to go to the Krabi immigration office to extend the VISA by another 30 days. If we go there early before the 28th, for example on the 21, would we lose 7 days from the VISA or would the new 30 day VISA start from the 28th/29th? Thank You in advance for your help and apologies if this question is a repeat.