DateTime operations

Extracting a Date and a Time out of a DateTime

A Date and a Time can be extracted out of a DateTime by deconstructing via pattern matching:

let (DateTime myDate myTime) = myDateTime

We can also use date and time:

date :: DateTime -> Date
time :: DateTime -> Time

Adjusting a DateTime

It’s possible to alter a DateTime directly by using modifyDate, modifyTime, modifyDateF and modifyTimeF:

modifyDate :: (Date -> Date) -> DateTime -> DateTime
modifyTime :: (Time -> Time) -> DateTime -> DateTime
modifyDateF :: forall f. Functor f => (Date -> f Date) -> DateTime -> f DateTime
modifyTimeF :: forall f. Functor f => (Time -> f Time) -> DateTime -> f DateTime

All these functions take as the first argument a function that modifies the specific DateTime component, a DateTime and return the new value. The latter two are convenience functions which let us work with modifying functions that produce a Functor. This is very useful, for example, in conjunction with the adjust function which produces a Maybe (which is a Functor!).

For example:

addThreeDays :: Date -> Maybe Date
addThreeDays = adjust (Days 3.0)

oldDateTime :: DateTime

newDateTime :: Maybe DateTime
newDateTime = modifyDateF addThreeDays oldDateTime

It’s also possible to use adjust directly:

adjust :: forall d. Duration d => d -> DateTime -> Maybe DateTime

This function adjusts a DateTime value with a Duration offset. Nothing is returned if the resulting date would be outside the range of valid dates.

Difference between two DateTime values

To calculate the difference between two DateTimes we can use diff:

diff :: forall d. Duration d => DateTime -> DateTime -> d

The result of diff is one of the types (Days, Hours, Minute, Seconds, Milliseconds) which implements the Duration type class. We need to help the compiler by specifying the type when it’s not clear from the context.

daysDiff :: Days
daysDiff = diff date1 date2

hoursDiff :: Hours
hoursDiff = diff date1 date2