I have a bunch of time variables in the format "day-month-year H:M", for example, 14-03-15 15:25. How do I correctly measure the difference between two time variables and output a duration type? 

```
std::string t1 = 14-03-15 15:25;
std::string t2 = 19-05-15 7:32;

template <typename Duration>
auto diff(std::string& t1, std::string& t2) {
   // How to do from here?
} 

auto ds = diff<std::chrono::seconds>(t1, t2);
auto dm = diff<std::chrono::minutes>(t1, t2);

```