fromNow() gives inconsistent results between UTC & local mode

[denchen]

Describe the bug
I've found a situation where fromNow() gives different results from MomentJS, and it gives different results depending on if you're in UTC mode or not.

Given the following:

let dutc = dayjs.utc(currentTimestamp37hrsAgo); // UNIX time for 37 hours before now
let mutc = moment.utc(currentTimestamp37hrsAgo);

console.log(`37 hours ago - DayJS (utc):     ${dutc.toISOString()}, fromNow(): ${dutc.fromNow()}`);
console.log(`37 hours ago - MomentJS (utc):  ${mutc.toISOString()}, fromNow(): ${mutc.fromNow()}`);

Output of the above:

37 hours ago - DayJS (utc):     2019-07-28T09:28:35.873Z, fromNow(): a day ago
37 hours ago - MomentJS (utc):  2019-07-28T09:28:35.873Z, fromNow(): 2 days ago

Here's a more extensive reproduction on Runkit

If it helps, my local timezone is PST (-07:00)

Expected behavior
It is my understanding that fromNow() should not "care" about timezones, and it's based strictly off of the difference in absolute time. fromNow() should give the same results, regardless of if we're using .utc() or not. I didn't check to see if toNow() also has the same problem, but I'd assume so.

Information

• Day.js Version: 1.8.15
• OS: MacOS 10.14.6
• Browser: Chrome 75.0.3770.142
• Time zone: -07:00 DST (Pacific Daylight Time)

[g1eny0ung]

Runkit Clone

Here is my result and seems that it works as normal. I'm in UTC+8 (Beijing).

By viewing documents and code, I found moment.js will compare the local time in the fromNow() method. The code

But day.js won't do some special handling. The fromNow() method in day.js will call the diff() Internally. The diff method will use different comparison methods for different units.

So, I think maybe In some special cases, the fromNow() will have inconsistent results.

I can't completely guarantee that I am right. Maybe need some discussion?
@denchen @iamkun @xxyuk

[denchen]

@g1eny0ung

The difference is that your timezone is ahead of UTC (+8), and my timezone is behind UTC (-7). If I manually change my laptop's timezone to +8, fromNow() gives correct results, but if I change my timezone to any timezone after UTC, even -1, then I'll get different results.

However, there's another way to replicate the difference without changing your timezone. Instead of subtracting 37 hours, add 37 hours. In other words:

let currentTimestamp37hrsAgo = currentTimestamp + (37 * 60 * 60 * 1000);

Then you'll get the different results in your Beijing timezone.

37 hours ago - DayJS:           2019-08-01T07:04:28.958Z,  fromNow(): in 2 days
37 hours ago - MomentJS:        2019-08-01T07:04:28.958Z,  fromNow(): in 2 days
37 hours ago - DayJS (utc):     2019-08-01T07:04:28.958Z,  fromNow(): in a day
37 hours ago - MomentJS (utc):  2019-08-01T07:04:28.958Z,  fromNow(): in 2 days

[g1eny0ung]

@denchen Got it. I will check the source code and try to find out where the problem lies.

[g1eny0ung]

@denchen #649

The problem is fromNow() method's internal diff method will calculate timezone offset when use dayjs.utc.

In order to maintain consistency, I use a boolean argument to avoid timezone comparison in relativeTime plugin change the fromNow() method to determine if this is in UTC mode.

[denchen]

Thank you for addressing this issue.

When can we expect a new release that includes this fix? A rough estimate would be fine.

[iamkun]

🎉 This issue has been resolved in version 1.8.16 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀