As noted by @ubonjoe, the visa is effective from it's issuing date. I am on my third METV and was delighted to learn that, at least in the US, it is now issued as an e-visa, which means being spared the expense and time of mailing the passport and other documents which added 10-12 days to the process. My e-visa was approved and downloaded in three days. Of course there's no knowing the future, but had I known I would have had an extra 10-12 days of validity from the time is my arrival in Thailand.