Mk1 is 1991 to 1998, inclusive of Phase1, 2 and 3. (the Phase3 was the most radical change).
Mk2 is 1998 to present, inclusive of Phase1 and 2. (the Phase2 is the same car with a fairly radical facelift).
But because the two 172s look so different, theyre commonly referred to as Mk1 and 2 (even though theyre Phases).