Hi,
For example, I suggest you input a date in A1 as a due date, then go to
"Format --> Conditional Formatting --> choose Formula Is in condition 1 -->
type =NOW()-A1>30 --> choose format button --> select Pattern tab --> choose
colour red --> OK --> OK.
So, if today's date is more than 30 days the date in A1, the cell will
change to colour red.
The same procedure applies to your second question if you input "Y" in A1
for accomplished task.
HTH