Yup. No RTC or anything on the board itself, and if there was then, as @archieroques rightfully says, it would take quite considerably longer to request the time from an external source than to call a Python-native function like
time.time(), which would render the whole process a little moot!
I’d say your best bet is to make sure your Pi’s clock is well calibrated, then figure out how long it takes to acquire a sample- roughly- and subtract that value from your Python-generated timestamp.
You’d get phase issues with this however, since the Enviro pHAT library configures the ADS1015 to generate 1600 samples per second, which is one sample every 625 microseconds, and you have no way of knowing how old a sample might be at the time you request and receive it, your actual sample age could vary by up to 625uS at a bare minimum. Couple this with the Pi being a multi-tasking OS, and you must add unpredictable delays onto that.
Anyway, I digress! It’s a hard problem without a simple answer.