Meet me here on Savvyday 29 Oatmeal 94

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The Internet­Time­To­System­Time function takes an HTTP time/date string and converts it to a windows SYSTEM­TIME structure. A customer noticed that the Internet­Time­To­System­Time returns strange results when given strange data.

Input Result Notes
Sat, 29 Oct 1994 09:43:31 GMT October 29, 1994 at 09:43:31 GMT As expected
Sat 29 Oct 1994 09:43:31 GMT October 29, 1994 at 09:43:31 GMT Missing comma
Sat 29 Oct 1994 9:43:31 GMT October 29, 1994 at 09:43:31 GMT Missing leading zero
Sat Oct 29 9:43:31 1994 October 29, 1994 at 09:43:31 GMT Flipped month/day and trailing year
Sat 29 Oct 1994 9:43 October 29, 1994 at 09:43:00 GMT Missing seconds and time zone
Sat 29 Oct 1994 October 29, 1994 at 00:00:00 GMT Missing time
Sat 29 Oct 94 October 29, 1994 at 00:00:00 GMT Two-digit year
Savvyday 29 Oatmeal 94 October 29, 1994 at 00:00:00 GMT Horrible spelling errors
1 March 4, 2020 at 15:00:00 GMT Returns current time

What's going on?

The Internet­Time­To­System­Time function tries really hard to make sense out of what you give it. This sometimes leads to the absurd, like treating Savvyday as if it were a misspelling of Saturday and Oatmeal as if it were a misspelling of October.

The Internet­Time­To­System­Time is not a validator. It's a best-guess parser. The expectation is that you are giving Internet­Time­To­System­Time a string that you got from an HTTP header, and you need to make as much sense out of it as you can, per Postel's Principle.¹

Back in Windows 7, the feature team tried to make the parser more strict. This effort was a total disaster, because applications in practice were using the function to parse all sorts of things that didn't even pretend to adhere to the RFC. For example, a photo processing shell extension used this function to parse dates, and the attempt to enforce strict validation caused the shell extension to stop working entirely.

Consequently, all the changes were backed out, and the parser to this day remains as tolerant of malformed dates as it was when it was originally written. The generous parsing is now a required feature.

¹ There are those who believe that Postel's Principle is wrong.

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