[release/3.0] Fix Abbreviated Genitive Month Names Parsing

[tarekgh]

In 3.0 we have fixed the formatting to use the abbreviated genitive month names when having "d" format specifier followed by "MMM". This fix is good as original formatting specs required but we needed to support the parsing when we have such genitive names.

[tarekgh]

https://github.com/dotnet/coreclr/issues/26370
#26379
https://github.com/dotnet/corefx/issues/2804
#21230

[tarekgh]

Description

In last December we have made a fix for the issue https://github.com/dotnet/corefx/issues/2804 allowing formatting dates with Genitive month names. The fix is good but we missed adding a support to the date parser to handle the genitive month names.
On Windows 10, there are 133 cultures with genitive names out of total 848 cultures.

Customer Impact

If the customer app dealing with date and time formatting and parsing functionality will be potentially broken if format a date with genitive months and then try to parse it back to DateTime. The parsing is going to throw exception. Some of the cultures uses date/time formatting pattern with genitive month names specifiers as the default format. For example, culture Ky uses a default short date pattern d-MMM yy produce a genitive month names in the date formatting and parsing such strings is going to throw exception.

Regression?

Yes, although the fix we did is not the real regression but the regression caused because we missed to add the parsing part with the fix.

Risk

The risk is low as we are going to allow the date parser to try the genitive month names so the failing case will start succeed.

Code Review

Stephen Toub