I have a bizarre problem here. In an app I'm making, a need to generate an array of consecutive date strings, where each date string is 'YYYY-MM-DD'. It worked fine for a long time, but then in November one of my users let me know that one date, 7 November 2021, was missing from the app.

But, it wasn't missing for anyone else. So I worked with him to narrow down the problem to the code below. It behaves differently if I or him run it in our Chrome consoles.

// Creates an array of 14 consecutive date strings
function datesInclGaps(date, hour) {
    const latest = toUTCDate(date, hour)
    const result = []
    for (let i = 0; i < 14; i++) {
        result.push(fromUTCDate(latest))
        latest.setDate(latest.getDate() - 1)
    }
    return result
}

// Creates a Date object with the given UTC date and the give hour.
const toUTCDate = (date, hour) => {
    const match = /(\d\d\d\d)-(\d\d)-(\d\d)/.exec(date)
    if (!match) throw new Error('Illegal date format: ' + date)
    const utc = new Date()
    utc.setUTCFullYear(+match[1], +match[2] - 1, +match[3])
    utc.setUTCHours(hour, 0, 0, 0)
    return utc
}

// Returns a date string YYYY-MM-DD from a Date obj.
// Date assumed to be in UTC.
const fromUTCDate = date => {
    const y = date.getUTCFullYear()
    const m = pad(date.getUTCMonth() + 1)
    const d = pad(date.getUTCDate())
    return `${y}-${m}-${d}`
}

const pad = (t, p = 2) => t.toString().padStart(p, '0')

console.log('0-hour')
console.log(datesInclGaps('2021-11-14', 0).join(', '))
console.log('12-hour')
console.log(datesInclGaps('2021-11-14', 12).join(', '))

The list of dates should both be the first 14 days of November. The 'hour' argument is how I finally 'fixed' the bug, since if I set it to 0, the day disappears for him, but if it is set to 12, it works fine. See the screenshots below. The light-mode is his output, with the error, and the dark mode is when I run it, with no error. Exact same simple plain Javascript run in the chrome console.

The reason I use the UTC versions is to make sure that timezones wouldn't matter. But... it is the only thing I can think of that might be a difference between us. I've racked my brain over this one but I cannot think of a single good reason why the code is behaving like this! Any insight would be useful!