For that class, of course using negative number would have done the trick. 

However, it is use afterward to compare to a limit date in the future, and using only the negative delta would not be enough since `NagiosW4lCheckMySQLUpdatedAt` inherit from `NagiosCheckMinThreshold` where `NagiosCheckMaxThreshold` is needed. I've done some factorisation on the other side of the pull request, and I personnally think this is the proper way to do things (instead of using a trick).

The other way to do that would have been to inherit the check from `NagiosCheckMaxThreshold` (and let the performance data computation been overloaded), use a negative number for the timedelta (as you said), overload the message, title and such. And even though the performance data of `oldest_updated_at` (reverse meaning) would be negative.

IMO, that would have been a misuse of the semantic of the classes, methods, perf_data...


On another side I don't know if it could have been implemented in a smarter way (after all only the `-` sign change).