A DateTime in C# is a value type, not a reference type, and therefore cannot be null. It can however be the constant DateTime.MinValue which is outside the range of Sql Servers DATETIME data type.
Value types are guaranteed to always have a (default) value (of zero) without always needing to be explicitly set (in this case DateTime.MinValue).
Conclusion is you probably have an unset DateTime value that you are trying to pass to the database.
DateTime.MinValue = 1/1/0001 12:00:00 AM
DateTime.MaxValue = 23:59:59.9999999, December 31, 9999,
exactly one 100-nanosecond tick
before 00:00:00, January 1, 10000
MSDN: DateTime.MinValue
Regarding Sql Server
datetime
Date and time data from January 1, 1753 through December 31, 9999, to an accuracy of one three-hundredth of a second (equivalent to 3.33 milliseconds or 0.00333 seconds). Values are rounded to increments of .000, .003, or .007 secondssmalldatetime
Date and time data from January 1, 1900, through June 6, 2079, with accuracy to the minute. smalldatetime values with 29.998 seconds or lower are rounded down to the nearest minute; values with 29.999 seconds or higher are rounded up to the nearest minute.
MSDN: Sql Server DateTime and SmallDateTime
Lastly, if you find yourself passing a C# DateTime as a string to sql, you need to format it as follows to retain maximum precision and to prevent sql server from throwing a similar error.
string sqlTimeAsString = myDateTime.ToString("yyyy-MM-ddTHH:mm:ss.fff");
Update (8 years later)
Consider using the sql DateTime2 datatype which aligns better with the .net DateTime with date range 0001-01-01 through 9999-12-31 and time range 00:00:00 through 23:59:59.9999999
string dateTime2String = myDateTime.ToString("yyyy-MM-ddTHH:mm:ss.fffffff");