Help to fill in a blank date by the setting it as the earliest date in a date slicer

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Hey,

I have an issue where I have blank Date data for a map. I need to have the blank data filled in the the earliest date I select on the date slicer so certain data always shows on the map when changing the date range.

I have currently put a temporary solution in here by merging the data with a date table for every day of the month, but this isn't clean or completely functional.

Any help would be great.