```
template <class Rep1, class Period1, class Rep2, class Period2>
struct common_type<chrono::duration<Rep1, Period1>, chrono::duration<Rep2, Period2>> {
using type = chrono::duration<common_type_t<Rep1, Rep2>, see below>;
};
```

The period of the duration indicated by this specialization of common_type shall be the greatest common divisor of Period1 and Period2. [ Note: This can be computed by forming a ratio of the greatest common divisor of Period1::num and Period2::num and the least common multiple of Period1::den and Period2::den. — end note ]

[ Note: The typedef name type is a synonym for the duration with the largest tick period possible where both duration arguments will convert to it without requiring a division operation. The representation of this type is intended to be able to hold any value resulting from this conversion with no truncation error, although floating-point durations may have round-off errors. — end note ]

```
template <class Clock, class Duration1, class Duration2>
struct common_type<chrono::time_point<Clock, Duration1>, chrono::time_point<Clock, Duration2>> {
using type = chrono::time_point<Clock, common_type_t<Duration1, Duration2>>;
};
```