As an example: Suppose you have dates in column B, starting in B2.
Suppose you require an entry in cell C2. With the cell pointer on C2,
click Format Conditional Formatting. Set Condition 1 to "Formula Is"
in the dropdown, and paste this formula into the formula box:
=AND(C2="",B2<TODAY()+1)
.... and set the formatting to color the cell red when conditions are
met.

The cell turns red when C2 is blank and B2, the date cell for that row,
is less than tomorrow's date. I always have problems getting
conditional formatting to work right the first time: if you use this
formula and don't get expected results, click Format Conditional
Formatting and make sure Excel didn't add extra " (double quotes) to
the formula: if it did, remove any extras and it should work.