Parsing "12/25/35" to a date using InvariantCulture (2-digit year)

When I use the InvariantCulture to parse a string, with both:

var christ1 = DateTime.Parse("12/25/35", CultureInfo.InvariantCulture);

and:

var christ2 = DateTime.ParseExact("12/25/35", "MM/dd/yy", CultureInfo.InvariantCulture);

the result depends on what computer the application runs on. On one machine I get the equivalent of:

December 25, 2035

while on another machine I get a date one century earlier, that is:

December 25, 1935

What is going on?

The so-called invariant culture is not so invariant in this case. It depends on both your OS version and the current user's regional settings in the OS.

In .NET terms, the property of interest is TwoDigitYearMax on the calendar, so inspect CultureInfo.InvariantCulture.DateTimeFormat.Calendar.TwoDigitYearMax or (new GregorianCalendar()).TwoDigitYearMax. Documentation of GregorianCalendar.TwoDigitYearMax override.

On a Windows 10 version 1809 ("October 2018 Update", "Redstone 5") machine where the user has not changed the regional settings (more on that below), the property TwoDigitYearMax is 2029, and as a consequence you get December 25, 1935.

But on Windows 10 version 1903 ("May 2019 Update", "19H1"), the same property returns 2049, and therefore your result is December 25, 2035.

However, the value depends not only on the Windows version. Also what the user inputs in regional settings, matters. So that is not really "invariant".

You can change the cutoff year in the Control Panel by:

1. Click Start, click "gear" icon on the left (Settings), and click Time & Language.
2. Click Region on the left.
3. Under Related settings, click Additional date, time, & regional settings.
4. (You are in Control Panel.) Click Region.
5. (In new Region window.) Click button Additional settings... near bottom of page, and in new window click Date tab.
6. In the Calendar group, adjust When a two-digit year is entered, interpret it as a year between.

In older Windows flavors, it may be (pasted from microsoft.com):

1. Click Start, point to Settings, and then click Control Panel.
2. Double-click the Regional Settings icon.
3. Click the Date tab.
4. In the When a two digit year is entered, interpret a year between box, type the cutoff year that you want, and then click OK.

Or you can use Windows Registry directly, go to key HKEY_CURRENT_USER\Control Panel\International\Calendars\TwoDigitYearMax, and for every relevant "value" in that key, keep the name component unchanged and edit the data component of the value to (the usual decimal string representation of) the cutoff year you desire. If the user has never changed this setting from the Control Panel, the two innermost keys (Calendars\TwoDigitYearMax) do not exist yet. Usual warnings about mingling with the registry apply.

Once you changed the year in the Control Panel, every .NET process that is started after you made these changes, will reflect your choice in the TwoDigitYearMax property reached through InvarantCulture!

So if you want consistent behavior across OS versions and user preferences, I think you cannot use InvariantCulture. Instead, something like this could work:

var tmp = (CultureInfo)(CultureInfo.InvariantCulture.Clone());
tmp.DateTimeFormat.Calendar.TwoDigitYearMax = 2039; // any cutoff you need
 // incorrect: tmp.Calendar.TwoDigitYearMax = 2039
var newInvariantCulture = CultureInfo.ReadOnly(tmp);

then use that newInvariantCulture instance whenever you parse dates with 2-digit years (you can put it in some static readonly field).

WARNING: Each CultureInfo x carries two distinct Calendars that may have different 2-digit year max, namely:

x.DateTimeFormat.Calendar.TwoDigitYearMax
  !=
x.Calendar.TwoDigitYearMax