You could combine DATEDIF function with Conditional Formatting.
If for example a passport lasts ten years and you want to be reminded 2
months before it expires. With an issue date of 26th November 1993 in A1, go
to Format -- Conditional Formatting.
Set Condtion1 to Formula Is. In the text box, enter
=DATEDIF(A1,TODAY(),"m")>117
(10 years = 120 months, so you want to know when it hits 118 months).
Click the Format button, select the Number tab, pick a colour and click OK.
Click OK again.
Now amend the date in A1 to 25th November 2003.
You can use the Format Painter tool to apply the conditional formatting to
other cells.
HTH,
Andy