They have a digital calendar with the date from 21 years ago.
It says:
To Purchase Tobacco Products You Must Have Been Born On Or Before This Date.
Thank you for showing you I.D.
Ah, easy to match the DOB on the I.D. presented to the date on the calendar.
Nothing to calculate, no mistakes.
They raised the age for smoking (cigarettes and vaping) in Maryland to 21 on October 1, so it's the same requirement as for drinking alcohol.
Luckily for me (even though I'm over 21), I don't really do either--definitely not the smoking, and the drinking limited primarily to the Kiddush prayer on Shabbat. ;-)
(Credit Photo: Andy Blumenthal)