While not “elegant”, you could simply calculate the difference between the current system date and the arbitrary value of days representing 2 years. This difference could then be assigned to a process variable and applied to an input column of the DMN table.
Depending upon how accurate this needs to be, you might have to account for leap years in your calculations. Personally, I prefer to use UNIX epoch type dates (number of seconds between now and 01-01-1970 00:00:00). This provides an absolute reference and is easily used in calculations.
You could also set up an input column to use the FEEL date and time function to determine if a date was inside the valid range represented by a period of current date - 2 years. The challenge here would be to automatically calculate a period of 2 years and set the input value to it. The date functions are described here: