```
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.

[ Note

: *end 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.

— ```
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();
```

```
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

: *end 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.

— ```
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>>;
};
```