You can conditionally format colors and fonts and borders, but not text, not in the same cell.

The only thing I can think of is you'll have to design a Worksheet_Change macro that will change the custom number formatting applied to the adjacent cells. Seems like a lot of work.

If you just use 3 cells, it's easy.

A1 = Days
B1 = 8
C1 =B1 & " " & A1