Add context function

[kossnocorp]

The next date-fns version will ship a feature that will allow using the specified context function to normalize the arguments and/or the result, enabling explicit time zones support:

import { tz } from "@date-fns/tz";
import { isSameDay } from "date-fns";

const date1 = "2024-04-10T07:00:00Z"; // Wed Apr 10 2024 00:00:00 GMT-0700 (Pacific Daylight Time)
const date2 = "2024-04-10T15:00:00Z"; // Wed Apr 10 2024 08:00:00 GMT-0700 (Pacific Daylight Time)
const date3 = "2024-04-11T07:00:00Z"; // Thu Apr 11 2024 00:00:00 GMT-0700 (Pacific Daylight Time)

isSameDay(date1, date2, { in: tz("America/Los_Angeles") })
//=> true
isSameDay(date1, date3, { in: tz("America/Los_Angeles") })
//=> false

It makes sense to add this feature to @date-fns/utc as well, so devs can use UTC the same way:

import { utc } from "@date-fns/utc";
import { isSameDay } from "date-fns";

const date1 = "2024-04-10T07:00:00Z";
const date2 = "2024-04-10T15:00:00Z";
const date3 = "2024-04-11T07:00:00Z";

isSameDay(date1, date2, { in: utc })
//=> true
isSameDay(date1, date3, { in: utc })
//=> false