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When I wrote billing systems for telecoms, Feb 31st was a valid date... after a fashion. Billing cycles were stored as a cycle period "monthly", "quarterly", etc, with a "bill day" offset which was a number 1-31. If your "bill day" was the 31st then in February you would be billed on the last day of the month along with the customers with bill days 28-30. It was rare to do actual time calculations treating the bill day as an actual date, but not considered particularly strange.

And there you have it, at least for one small corner of the software world.

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