```
template <class Rep1, class Period1, class Rep2, class Period2>
struct common_type<chrono::duration<Rep1, Period1>, chrono::duration<Rep2, Period2>> {
typedef chrono::duration<common_type_t<Rep1, Rep2>,
```

*see below*> type;
};

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>> {
typedef chrono::time_point<Clock, common_type_t<Duration1, Duration2>> type;
};
```

The common type of two time_point types is a time_point with the same clock as the two types and the common type of their two durations.