You can use the DateTime.MaxValue property to retrieve the largest possible date, dependent on the system. If you are having problems reading dates due to overflow, you can use the DateTime.TryParse() methods to attempt a read of the value without throwing an exception.
UPDATE:
Martinho is correct. From MSDN:
Some calendars, such as the
UmAlQuraCalendar, support an upper
date range that is earlier than
MaxValue. In these cases, trying to
access MaxValue in variable
assignments or formatting and parsing
operations can throw an
ArgumentOutOfRangeException. Rather
than retrieving the value of
DateTime.MaxValue, you can retrieve
the value of the specified culture's
latest valid date value from the
System.Globalization.CultureInfo.DateTimeFormat.Calendar.MaxSupportedDateTime
property.
So, use the System.Globalization.CultureInfo.DateTimeFormat.Calendar.MaxSupportedDateTime
field to get the maximum date value supported by the system.