Date-fns: Rename parse's backupDate argument to referenceDate

[lukewlms]

The naming "backupDate" seems strongly to indicate that this date will be used as a "backup" when parsing fails. However this is not what happens - instead, failure returns Invalid Date.

It seems to me that this data parameter is being used as a reference where needed. E.g. if a time is missing, it can be gotten from the reference; if the year is 2 digits and therefore unclear, we reference the date passed in to see what year is most likely.

You can probably see where I'm going here. 😄 To keep from causing confusion with the API I'd suggest referenceDate is clearer naming.

"backupDate" is never fully used as a backup because there's no case where all of its subvalues would be used.

(I checked out https://github.com/date-fns/date-fns/issues/1219 which is where this naming comes from.)

We love DateFns and evangelize it, thanks!

[icleolion]

Created a PR for this as I agree. backupDate was initially confusing for me too. A different name could potentially be clearer. The PR changes it to referenceDate as you've suggested, although I was tempted to go with contextDate instead.

[kossnocorp]

I like the idea. The reference date is clearer. Thanks!

[lukewlms]

https://github.com/date-fns/date-fns/pull/1607 merged to master.

Thanks @icleolion!