Not only do you need to specify the precise dates of recurrences, but you also need to map time intervals to fractions of a year (e.g. if I owe you $1mm a year, is 'one month' equal to 1/12 of a year, or do I count the actual numer of days in between? What about leap days, etc.). Getting it wrong even slightly means you get all the cashflows wrong.

Welcome to the joy of 'day count conventions' [1] and 'date rolling' [2].

[1] https://en.wikipedia.org/wiki/Day_count_convention [2] https://en.wikipedia.org/wiki/Date_rolling

Funny thing is these have all probably be reimplemented from scratch at every single bank.