How to stop Excel changing dates to numbers

It's not entirely clear, but it seems like you have a formula like this:

=A2&"_"&B2&"_"&C2

... where A2 is a patient number, B2 is a kit number, and C2 is a date. You expect C2 to be pulled in with the same formatting but it's showing up as a number, probably 44403 if you're on Windows and the date is 26-JUL-2021. This is because Excel treats dates (and times) as numbers. For windows, it counts the days from 01-JAN-1900. If you reference a date in a formula, it uses the number. There's a way to force it to whatever format you want, though:

=A2&"_"&B2&"_"&UPPER(TEXT(C2,"dd-mmm-yyyy"))

TEXT formats a number as desired and returns the result as a string. UPPER will make it return JUL instead of Jul.