in c#, you may use an abstract class for Tools and a derivated class for each of the tools.

homeboy commented: So, I just create classes abstract for Tools (Electric, Hydraulic & Mechanic) and derivated class for Started, Stopped & Stopped urgently. Am I right ? +0

With .Net, you have two types of variables that will be usefull to solve your problem :
DateTime and TimeSpan. The last one is the difference between two DateTime. So you can write something like :
DateTime t1;
DateTime t2;
TimeSpan delta = t2 - t1;

TimeSpan embarks a bunch of methods to convert into differents units.