Is a multi-entry visa valid from the date of issue or the date of entry into Thailand?

All visas are valid from the day they are issued, and can be used until the end date on the visa.

The difference between single-entry and multi-entry is that once you enter with the single-entry visa, it's done, and can no longer be used.

With multi-entry, you can enter an unlimited number of times between the day it is issued and the end date on the visa.