# Interoperability with the `Dates`

standard library

Julia's `Dates`

standard library provides data types for representing specific points in time `Date`

/`DateTime`

and differences between them, i.e., periods. Unitful provides methods for using period types from the `Dates`

standard library together with `Quantity`

s.

## Support for `Dates.FixedPeriod`

s

The `Dates.FixedPeriod`

union type includes all `Dates.Period`

s that represent a fixed period of time, i.e., `Dates.Week`

, `Dates.Day`

, `Dates.Hour`

, `Dates.Minute`

, `Dates.Second`

, `Dates.Millisecond`

, `Dates.Microsecond`

, and `Dates.Nanosecond`

. These types can be converted to `Quantity`

s or used in place of them.

`Dates.Year`

does not represent a fixed period and cannot be converted to a `Quantity`

. While Unitful's `yr`

unit is exactly equal to 365.25 days, a `Dates.Year`

may contain 365 or 366 days.

Each `FixedPeriod`

is considered equivalent to a `Quantity`

. For example, `Dates.Millisecond(5)`

corresponds to the quantity `Int64(5)*u"ms"`

. A `FixedPeriod`

can be converted to the equivalent `Quantity`

with a constructor:

`Unitful.Quantity`

— Method`Quantity(period::Dates.FixedPeriod)`

Create a `Quantity`

that corresponds to the given `period`

. The numerical value of the resulting `Quantity`

is of type `Int64`

.

**Example**

```
julia> using Dates: Second
julia> Quantity(Second(5))
5 s
```

In most respects, `FixedPeriod`

s behave like their equivalent quantities. They can be converted to other units using `uconvert`

, used in arithmetic operations with other quantities, and they have a `unit`

and `dimension`

:

```
julia> using Dates: Hour
julia> p = Hour(3)
3 hours
julia> uconvert(u"s", p)
10800 s
julia> p == 180u"minute"
true
julia> p < 1u"d"
true
julia> 5u"s" + p
10805 s
julia> 210u"km" / p
70.0 km hr^-1
julia> unit(p) === u"hr"
true
julia> dimension(p)
𝐓
```

Conversely, a `FixedPeriod`

can be created from a quantity using the appropriate constructor, `convert`

, or `round`

methods. This will fail (i.e., throw an `InexactError`

) if the resulting value cannot be represented as an `Int64`

:

```
julia> using Dates: Day, Hour, Millisecond
julia> Millisecond(1.5u"s")
1500 milliseconds
julia> convert(Hour, 1u"yr")
8766 hours
julia> Day(1u"yr")
ERROR: InexactError: Int64(1461//4)
[...]
julia> round(Day, 1u"yr")
365 days
```

## Support for `Dates.CompoundPeriod`

s

The `Dates`

standard library provides the `Dates.CompoundPeriod`

type to represent sums of periods of different types:

`julia> using Dates: Day, Second`

`julia> Day(5) + Second(1)`

`5 days, 1 second`

`julia> typeof(ans)`

`Dates.CompoundPeriod`

Unitful provides facilities to work with `CompoundPeriod`

s as long as they consist only of `FixedPeriod`

s. Such `CompoundPeriod`

s can be converted to `Quantity`

s using `convert`

, `uconvert`

, or `round`

:

```
julia> using Dates: Day, Second
julia> p = Day(5) + Second(1)
5 days, 1 second
julia> uconvert(u"s", p)
432001//1 s
julia> convert(typeof(1.0u"yr"), p)
0.01368928562374832 yr
julia> round(u"d", p)
5//1 d
julia> q = Month(1) + Day(1) # Month is not a fixed period
1 month, 1 day
julia> uconvert(u"s", q)
ERROR: MethodError: no method matching Quantity{Rational{Int64},𝐓,Unitful.FreeUnits{(s,),𝐓,nothing}}(::Month)
[...]
```

However, not all operations that are defined for `FixedPeriod`

s support `CompoundPeriod`

s as well. The reason for that is that a `CompoundPeriod`

does not correspond to a specific unit:

```
julia> p = Day(365) + Hour(6)
365 days, 6 hours
julia> unit(p) # A CompoundPeriod does not have a corresponding unit ...
ERROR: MethodError: no method matching unit(::Dates.CompoundPeriod)
[...]
julia> dimension(p) # ... but it does have a dimension
𝐓
julia> Quantity(p) # As a result, there is no Quantity type associated with it ...
ERROR: MethodError: no method matching Quantity(::Int64)
[...]
julia> T = typeof(1.0u"hr"); T(p) # ... but it can be converted to a concrete time quantity
8766.0 hr
```

Consequently, any operation whose result would depend on the input unit is not supported by `CompoundPeriod`

s. For example:

`+(::Quantity, ::CompoundPeriod)`

and`+(::CompoundPeriod, ::Quantity)`

error, since the unit of the result depends on the units of both arguments.`div(::Quantity, ::CompoundPeriod)`

and`div(::CompoundPeriod, ::Quantity)`

work, since the result is a dimensionless number.`mod(::CompoundPeriod, ::Quantity)`

works, but`mod(::Quantity, ::CompoundPeriod)`

does not, since the second argument determines the unit of the returned quantity.