I hope you are on a different time zone than me - it's just after 3
here!

Highlight the cells D2 to D_whatever, with D2 as the "activecell", then
Format | Conditional Format then select Formula Is rather than Cell
Contents and enter this formula:

D2C2

then click the Format button and choose the effect/font/colour that you
wish and OK.

"_whatever" is the bottom cell that you use in column D (depends how
many employees you have). Then you can copy and paste this range of
cells to G2 downwards, then J2 downwards etc for each month of the
year. You could also use the Format Painter for this last part.

Hope this helps.

Pete