Calculate differnce between 2 dates, but if 1 date cell is blank leave cell blank

I am trying to calculate the difference between two dates, today's date and a user entered date.
Where there is data, this formula seems to work

Formula:

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$C$1 is today's current date.
If U4 is blank I want it to subtract T4 by todays date.
Seems to work except when there is no date in T4 or U4 it gives a number of 41801.
I am trying to figure out how to have it lever the cell empty if no data is in T4 or U4.

I found some help and went with this...

Formula:

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Seems to be doing the job