There are a few ways you could do this.
May 5th, 1999
5th of May, 1999
May the Fifth, 1999
The Fifth of May, 1999
If your wanting it completely written out:
May the Fifth, Nineteen Ninety-nine
The Fifth of May, Nineteen Ninety-nine
Although, this would be very uncommon to use.
See : https://ccm.net/forum/affich-341723-convert-a-date-figure-into-word-in-excel#3
Hope this helps.