If you want to include a date value in a cell using CONCATENATE to combine text and other cells, you need to include the TEXT function and include the format of the date you want.

A B C < C function >
1 Stephen 18/12/1946 Stephen’s birthday is 18/12/1946 =CONCATENATE(A1,”‘s birthday is “,TEXT,(B1,”dd/mm/yyyy”))
1 George 14 May 1944 George’s birthday is 14-May-1944 =CONCATENATE(A2,”‘s birthday is “,TEXT,(B2,”dd-mmm-yyyy”))
1 Christopher 30-7-70 Christopher’s birthday is 30 December 1970 =CONCATENATE(A3,”‘s birthday is “,TEXT,(B3,”dd mmmm yyyy”))


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