fix differenceInDays across DST (#533)

[dcousens]

Resolves #533

I'm happy to make changes where necessary, this was simply the simplest solution for the primitives.

[dcousens]

For reference, BEFORE THIS PATCH the differenceInDays function returned different results depending on your timezone.

Please note, I have used ISO-8601 timestamps to avoid ambiguity.

const { differenceInDays } = require('date-fns')

const a = new Date('2019-10-05T15:00:00.000Z') // for TZ=Australia/Sydney, this is 1 hour before DST
const b = new Date('2019-10-06T14:00:00.000Z') // for TZ=Australia/Sydney, this is 1 day  after DST
const c = new Date('2019-10-07T14:00:00.000Z') // for TZ=Australia/Sydney, this is 2 days after DST

console.log(differenceInDays(c, b)) 
console.log(differenceInDays(b, a))
console.log(differenceInDays(c, a))

NOTE: These results are from before this patch.

For TZ=Australia/Sydney

1
1
2

For TZ=UTC

1
0
1

[dcousens]

@kossnocorp anything I need to do to help this?

[dkozickis]

Would be cool if you can add a bit info about the case fixed in this PR to the function description?

[dkozickis]

Please move 86400000 into a var MILLISECONDS_IN_DAY

[dcousens]

@dkozickis moved

[dcousens]

Would be cool if you can add a bit info about the case fixed in this PR to the function description?

What about

This function returns the difference in days as an integer whole number of 24 hour periods between two timestamps,  and thereby ignores DST changes.

[dkozickis]

@dcousens sounds logical to me