The nomenclature of 1 in x years is based on statistics and equates to the chance, 1/x, that in a given year this event will happen. You don't need 1300 years of data to say this was a 1 in 1300 year event, but you can say that this event had a 1/1300=7.6^-4 chance of happening in a given year. And that can be calculated based on any amount of data. Of course, more data will give a higher confidence to the statement.