isToday(new UTCDate()) => false

[tilowestermann]

There is an issue with the isToday function

The problem

💻 Code demonstrating the problem

import { UTCDate } from '@date-fns/utc';
import { isToday } from 'date-fns';

isToday(new UTCDate());
//=> false

Demo: https://stackblitz.com/edit/stackblitz-starters-bdluhl?file=src%2Fmain.ts

🙁 Actual behavior

new UTCDate() is not today

🙂 Expected behavior

new UTCDate() is today

Debug information

• date-fns version: 3.4.0 (@date-fns/utc: 1.2.0)
• Browser/Node.js version: Edge
• Your timezone: UTC+1
• Your current time: 15:00

[fturmel]

date-fns is determining today with Date.now(). While this gives us the correct timestamp, it isn't wrapped in a UTCDate object. This causes the later startOfDay() calculation (used by isSameDay()) to fail because the start of the day isn't the same in UTC vs the local timezone.

There might be a need to audit all date-fns functions and use constructFrom() to create all "now" dates with a matching constructor when they are paired with a user-defined date. There are at least 8 functions affected by this interop problem.

A fix for isToday() could look like this:

// currently:
return isSameDay(date, Date.now()); 

// fix:
return isSameDay(date, constructFrom(date, Date.now()));

It would be a good idea to devise a testing strategy for this type of issue between the two projects.

[kossnocorp]

@fturmel, you're right about that. Thank you for pointing the way. I will review it and ship a fix.

[kossnocorp]

Fixed via b1a5eb5

[kossnocorp]

Shipped with date-fns@3.5.0