Dates in Excel ARE large numbers like 43567.... then Excel formats that number to display as a date format you choose.

If you try to concatenate a date cell, it will reveal the actual underlying number as it is converted to text. Concatenation returns a text string, FYI.

But now that you know that, you can use the TEXT() function to format the date 'text' in the date 'format' that you want.

=TEXT(D1, "dd mmmm yyyy ") & D2