If your required context is for a year only why not use a integer field instead of a date field? However, if you want to assign a Year only within a date field, then you would have to chose a default value for your Month and day as Darren Wiens link indicates.
If changing your date field to an integer is not an option you can use the the following script and substitute your own default month/day....
CDATE("1/1/2016")
Note in vbscript if memory serves right Date() returns the system date......