I am not sure if I got it.
I understand there is a reference value say "100" that occurred on 1st March
and you want to know if the certain results on A1 get over "100" the date is
registered otherwise the get the date of the "100" value.
On B2 put the date of the A2 value. And on the A4 =IF(A1A2,TODAY(),B2)

If you want to save the maximum of several tries that change the result
automatically I don't think it will be possible. A formula will always
change, as far as I know and you cannot freeze it if some conditions are met.
In any case, the value on A2 would have to take the new maximum so that the
sheet could continue to be used, isn't it?

Now, you
....either record the maximum records on a list somewhere, and link A2 and B2
to that list;
....or change manually the 100 to the new max say 120 and the date with the
copy values.
....or I have no idea of what I am talking about and will leave it...
b&a,n__o