What do you mean “won’t save”? Is there some error?
I’m not a computer so just looking at some YAML without the error message telling me what’s wrong is rarely productive.
Where did you come up with “datepicker” for the type? I think it should be just date. Clicking on the link under the “type” field shows: Inputs / Form Inputs | Framework7 Documentation
What type of Item is StartDatum?
Look at DateTime Standalone Widget and DateTime List Item for working examples using a full date and time picker. You should be able to just change the type to date.