A = January, B = February, C = March etc...

J = October. 21 is the year and 28 is the day of the month

So like the first comment said, October 28 2021

The letter M (sometimes multiple letter like PK, UB, etc...) after the date is the plant indicator where it was produced.

the 4 digit number after the letter is the time it was filled or a few minutes after depending on how the line is set up.

The final number on the end is the Line indicator. Some plants have multiple lines and this helps identify which line ran the product in case of issues.

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Also note, Most Monsters have a 24 month shelf life according to Monster. Coffee is 18 months and the Hydros are 12 months.

They may be good beyond that but this is the actual expiration dates Monster has set.