Convert a two digit year to a four digit year

Given that there are people alive now born before 1950, but none born after 2010, your use of 50 as the flipping point seems broken.

For date of birth, can you not set the flip point to the 'year of now' (i.e. 10) in your app? Even then you'll have problems with those born before 1911...

There's no perfect way to do this - you're creating information out of thin air.

I've assumed DOB = date-of-birth. For other data (say, maturity of a financial instrument) the choice might be different, but just as imperfect.

I think Java has a good implementation of this:

http://java.sun.com/javase/6/docs/api/java/text/SimpleDateFormat.html#year

People rarely specify years far into the future using a two-digit code. The Java implementation handles this by assuming a range of 80 years behind and 20 years ahead of the current year. So right now, 30 would be 2030, while 31 would be 1931. Additionally, this implementation is flexible, modifying its ranges as time goes on, so that you don't have to change the code every decade or so.

I just tested, and Excel also uses these same rules for 2-digit year conversion. 1/1/29 turns into 1/1/2029. 1/1/30 turns into 1/1/1930.

The .NET framework has a method that does exactly what you want:

int fourDigitYear = CultureInfo.CurrentCulture.Calendar.ToFourDigitYear(twoDigitYear)

That way you will correctly adhere to current regional settings as defined in Control Panel (or group policy):

You can also use the DateTime.TryParse method to convert your date. It uses the current culture settings to define the pivot year (in my case it is 2029)

DateTime resultDate;
Console.WriteLine("CultureInfo.CurrentCulture.Calendar.TwoDigitYearMax : {0}", System.Globalization.CultureInfo.CurrentCulture.Calendar.TwoDigitYearMax);
DateTime.TryParse("01/01/28", out resultDate);
Console.WriteLine("Generated date with year=28 - {0}",resultDate);
DateTime.TryParse("01/02/29",out resultDate);
Console.WriteLine("Generated date with year=29 - {0}", resultDate);
DateTime.TryParse("01/03/30", out resultDate);
Console.WriteLine("Generated date with year=30 - {0}", resultDate);

The output is:

CultureInfo.CurrentCulture.Calendar.TwoDigitYearMax : 2029

Generated date with year=28 - 01/01/2028 00:00:00

Generated date with year=29 - 01/02/2029 00:00:00

Generated date with year=30 - 01/03/1930 00:00:00

If you want to change the behavior you can create a culture with the year you want to use as pivot. This thread shows an example

DateTime.TryParse century control C#

But as martin stated, if you want to manage a time period that spans more than 100 year, there is no way to do it with only 2 digits.