Modifying (.add(), .subtract(), etc.) a dayjs instance created in UTC corrupts it

[marnixhoh]

Setting timezone explicitly

dayjs.tz(0, 'UTC').format()

'1970-01-01T00:00:00Z' - Result
'1970-01-01T00:00:00Z' - Expected

---

dayjs.tz(0, 'UTC').add(0,'minutes').format()

'1969-12-31T23:00:00Z' - Result
'1970-01-01T00:00:00Z' - Expected

---

dayjs.tz(0, 'UTC').add(1,'minutes').format()

'1969-12-31T23:01:00Z' - Result
'1970-01-01T00:01:00Z' - Expected

Setting default timezone

The above results are identical, when omitting 'UTC' and setting UTC as default:

dayjs.tz.setDefault('UTC')

dayjs.tz(0).format()

'1970-01-01T00:00:00Z' - Result
'1970-01-01T00:00:00Z' - Expected

---

dayjs.tz(0).add(0,'minutes').format()

'1969-12-31T23:00:00Z' - Result
'1970-01-01T00:00:00Z' - Expected

---

dayjs.tz(0).add(1,'minutes').format()

'1969-12-31T23:01:00Z' - Result
'1970-01-01T00:01:00Z' - Expected

---

Information

• Day.js Version 1.11.5
• OS: Mac
• Browser: Chrome

[BePo65]

I could reproduce the issue; working on a solution / pr.

[maulik1729]

@BePo65 found any solution for this?
Same problem for:
dayjs("2022-09-28").tz("UTC", true).format("YYYY-MM-DD HH:mm:ss");
moment("2022-09-28").tz("UTC", true).format("YYYY-MM-DD HH:mm:ss");
Giving different output.

Thanks.

[BePo65]

Yes I tried every combination and found that the problem is the utc time zone (even as gmt e.a.) I am working on it for the last 2 weeks, but i could not yet solve all combinations. At the moment I just have to fix the .tz('UTC') variant.

Please be patient as it will take me for sure a few more days; but I won't surrender 😃

[maulik1729]

dayjs(data).tz('UTC', true).format('YYYY-MM-DD HH:mm:ss'): incorrect output
dayjs(data).utc(true).format('YYYY-MM-DD HH:mm:ss'): correct output

@BePo65 this gives different output shouldn't we use utc implementation when timezone has offset of zero?

proto.tz = function (timezone = defaultTimezone, keepLocalTime) { 
        const oldOffset = this.utcOffset();
        const date = this.toDate();
        const target = date.toLocaleString("en-US", { timeZone: timezone });
        const diff = Math.round((date - new Date(target)) / 1000 / 60);
        const offset = -Math.round(date.getTimezoneOffset() / 15) * 15 - diff;
        if(offset === 0){                           
            return this.utc(keepLocalTime);    //Please check this    
        }
        let ins = d(target)
            .$set(MS, this.$ms)
            .utcOffset(offset, true);
        if (keepLocalTime) {
            const newOffset = ins.utcOffset();
            ins = ins.add(oldOffset - newOffset, MIN);
        }
        ins.$x.$timezone = timezone;
        return ins;
    };

[BePo65]

good idea; trying to use it in my pr.