How to I set a sharepoint online date column to be empty using update items action?

In NWC using the update items action I want to set the value of a date column to be empty.

I have an assets list which staff check out and check in items.

When an asset is checked out I set the custom date column 'Check Out Time' to the current data/time.

When the asset is checked back in I want to clear out the date/time from the 'Check Out time' column.

In the update items action it will not let me publish the workflow when I leave the value for the field blank.

I cannot create a variable of date/time without setting an initial value.

If I try a text variable which is empty the workflow errors on executing the update items action.

Anyone come across this before and found a solution?