Writing an Expression 

SAP MDM product calender starts from 1/1/1600.  So Date counter starts from 0 on 1/1/1600. As you know MDM Console does not provide any functionality to set default value in a Date field. But you can achieve this using an assignment expression in Data Manager.

I would not talk about how to make a date field in MDM Console and run an assignment in Data manager.

Lets take an example to set a default value of 1/1/2000 in a date field. 

You just need to write an expression on the basis of below given formulas:

Total Number of Year = Year given in default value - 1601.

i.e. 2000 - 1601 = 399.

Now, consider number of Leap years as well = 399/4 = 99 Years

So, Total Number of Days = (99 * 366) + (300 * 365) = 36234 + 109500 = 145734

Make an assignment on Date field and write the number of calculated days in the expression.

Note: Sometimes you need to adjust the value by +/- 5 days from the actual calculated days.

Run the assignment on a record in Data Manager. You will get the desired result.

I hope this blog will help to set default value in MDM Date fields as per the business requirements.