Hello Alberto,

Array-enter into D1:
=MAX(FREQUENCY(IF($A$1:$A$4=A1,$B$1:$B$4),IF($A$1: $A$4=A1,$C$1:$C$4)))
and copy down to D4.

Please notice that you need to array-enter this (enter with CTRL+SHIFT
+ENTER, not only with ENTER!).

All resulting numbers should be 1. Any higher number than 1 indicates
an overlapping range for the corresponding date code.

Regards,
Bernd