Serial Number to Date lookup for Seamaster 300M?

You can’t date a modern Omega watch based on those online serial # charts - as you saw, they are generally wrong! All you can tell is the sold date if you have the warranty card AND it’s filled in properly. A lot are left blank by the seller. Assuming you are sure the watch is real, then the best you’re going to do is it was manufactured sometime between 2018 and yesterday.

Actually there are discernible patterns that can be used to date even modern Omegas but they run within a model line so unless you have lots of data points to use you are flying blind. SMP Seamasters have serials in the 80-83m range going back as far as year ~2000, prior to that they used 55-61m serials and there was a jump between the ranges. If you have seen enough you can extrapolate a date. There was a similar pattern with Lemania and F Piguet based Speedmasters, those sat between 77m to 79m for a similar ~20 year period of time but recently jumped into another range. Those too can be roughly dated by where in the range they fall if you have enough comparables to work with. I realise that is contradicted by the 2220.80 datapoint above which has a higher number but there are always wrinkles to account for!