isSame() producing strange results

dayjs is making me feel like I’m going insane!

dayjs('2020-10-13T03:30:00.000Z').isSame('2020-10-12T04:53:43.427Z', 'day') // true
dayjs('2020-10-13T03:30:00.000Z').tz('UTC').isSame('2020-10-12T04:53:43.427Z', 'day') //false

Both dates are in UTC, so why is the first query incorrectly saying they’re in the same day?

Does dayjs() assume a local timezone and isSame doesn’t (or vice versa)?

[syntaxyz]

Have fun.

console.log("Default behaviour:")
const s = dayjs.tz("2022-03-01 15:00", 'Europe/Amsterdam')
const e = dayjs.tz("2022-03-01 15:00", 'Europe/Amsterdam')
console.log("issame?", s.isSame(e)) // false
console.log("isAfter?", s.isAfter(e)) // true
console.log("isBefore?", s.isBefore(e)) // false

dayjs.prototype.isSame = function(that: Date, units: string) {
    const other = dayjs(that).utc()
    return this.utc().startOf(units) <= other && other <= this.utc().endOf(units)
}

dayjs.prototype.isAfter = function(that: Date, units: string) {
    return dayjs(that).utc() < this.utc().startOf(units)
}

dayjs.prototype.isBefore = function(that: Date, units: string) {
    return this.utc().endOf(units) < dayjs(that).utc()
}

console.log("Overridden behaviour")
console.log("issame?", s.isSame(e)) // true
console.log("isAfter?", s.isAfter(e)) // false
console.log("isBefore?", s.isBefore(e)) // false

[Aloshh]

@iamkun, we are also struggling with this issue. Let me explain my point by giving an illustrative example. We have two dayjs objects, namely: date and breakStartTest, both of them have the the following value 2021-11-17 at 00.00 (local time) or 1637103600000 ms (see query below).

date.isSame(breakStartTest, 'day') //false

Moreover, we are in the CEST timezone. If both dayjs objects assume a local timezone then we expect isSame to return true, meaning that both occur on the same day. However, in our case isSame returns false. Our theory is that the date object (see code above) assumes local time while the breakStartTest object assume UTC. This theory is further confirmed by the query below in which we converted the date object to UTC:

date.utc().isSame(breakStartTest, 'day') //true

We have performed several tests, all of them gave the same result. I have attached an image below to illustrate my point (please excuse me for the artwork)

We kindly ask for clarification, thanks!