Hi Max,
I just tested this in our project running on Mendix 9.16.0. After choosing a date, a small button appears to empty the date again:
In case this isn't available for you, I would suggest updating the “Data Widgets” add-on from the marketplace first. It might be possible that you're using an outdated version of Datagrid 2.
If that doesn't fix the issue, my recommendation would indeed be upgrading Mendix.
EDIT: I just noticed that I probably missunderstood your question. However I just tried setting a default value and emptyíng again in the modeler, which also seems to work. I would try updating Data Widgets first, and if that doesn't resolve the issue, upgrading the Mendix version would be the next step.