A DateTime object cannot contain just a year, if you just wanna store a year, then you can probably just use a int variable to do that, and if (not recommended) you want to store only year in DateTime object do something like..
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DateTime dt = new DateTime(datetimepicker1.Value.Year, 1,1);
this will although put month and date as 1st January
Hmm I think I would have suggested that you use the DateTime.Parse Method or the DateTime.TryParse Method as apposed to using the DateTime constructor to create a new DateTime object, supplying the first day and month of the year....
The DateTime.Parse(String) method tries to convert the string representation of a date and time value to its DateTime equivalent.