```
template <class Rep> struct treat_as_floating_point
: is_floating_point<Rep> { };
```

The duration template uses the treat_as_floating_point trait to help determine if a duration object can be converted to another duration with a different tick period. If treat_as_floating_point_v<Rep> is true, then implicit conversions are allowed among durations. Otherwise, the implicit convertibility depends on the tick periods of the durations. [ Note: The intention of this trait is to indicate whether a given class behaves like a floating-point type, and thus allows division of one value by another with acceptable loss of precision. If treat_as_floating_point_v<Rep> is false, Rep will be treated as if it behaved like an integral type for the purpose of these conversions. — end note ]

```
template <class Rep>
struct duration_values {
public:
static constexpr Rep zero();
static constexpr Rep min();
static constexpr Rep max();
};
```

The duration template uses the duration_values trait to construct special values of the durations representation (Rep). This is done because the representation might be a class type with behavior which requires some other implementation to return these special values. In that case, the author of that class type should specialize duration_values to return the indicated values.

```
static constexpr Rep zero();
```

Returns: Rep(0). [ Note: Rep(0) is specified instead of Rep() because Rep() may have some other meaning, such as an uninitialized value. — end note ]

```
static constexpr Rep min();
```

```
static constexpr Rep max();
```

```
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>>;
};
```