I still found it strange that Mx6.10.3 was not retracted because this bug is in my opinion not a trivial one. Make a hotfix microflow that retrieves all the objects with this date (and think about possible scenario's where that date is correct!) and then just set the attribute to empty. That is all it takes.
And why stick to 6.10.3. My advise would be to upgrade to a newer version.
See this microflow as example: https://modelshare.mendix.com/models/bb76186d-e3ae-4fd6-b5ce-259a11efdc21/change-date-to-empty
One option (which may not be best idea), would to use an "On leave" event on the date picker widget. In the microflow, do a check for the date is equal to '01-01-1970', if its true , use the change activity to set the attribute to empty, if its false don't do anything.
Alternatively, you can check out these widgets
Hope this helps, I don't know much about this bug.
I found a solution! :)
I compared entered date and the date with value '11-02-1971'. If the entered date is before this value, then change it's value to 'empty'. This didn't work because on change action I didn't put commit. After I did that, everything works properly.