计算时区偏移：实现用户本地8点邮件发送的时区筛选问题

Let's break down why your original function fails and how to fix it. The core issue is that your current calculation doesn't account for the 24-hour cyclic nature of time and the valid range of real-world UTC offsets (which spans from -12 to +14, covering edge cases like UTC+14/Line Islands and UTC-12/Baker Island).

The Correct Approach #

We need to find all UTC offsets where currentUTC + offset ≡ targetLocalHour (mod 24)—meaning adding the offset to the current UTC hour lands us at the target local time—while ensuring those offsets fall within the valid range of real-world timezones. Here's a revised function that handles this properly:

function getValidUTCOffsets(targetLocalHour, currentUTCHour) {
    const validOffsets = [];
    // Calculate the base offset using the core formula
    let baseOffset = targetLocalHour - currentUTCHour;
    
    // Check the base offset and its ±24 equivalents (since adding/subtracting 24h gives the same local time)
    const possibleOffsets = [baseOffset, baseOffset + 24, baseOffset - 24];
    
    // Filter to only keep offsets that exist in real-world timezones
    for (const offset of possibleOffsets) {
        if (offset >= -12 && offset <= 14) {
            validOffsets.push(offset);
        }
    }
    
    // Remove duplicates (in case the base offset is already valid and ±24 isn't needed)
    return [...new Set(validOffsets)];
}

How It Works #

1. Base Offset Calculation: Start with the same core logic targetLocalHour - currentUTCHour to get a starting point.
2. Cyclic Adjustment: Since adding or subtracting 24 hours to an offset results in the same local time (a full day cycle), we check these three possible offset values.
3. Validation: We only keep offsets that fall within the real-world valid range (-12 to +14). This eliminates invalid values like -13 or -15, and captures both valid offsets when they exist (like -10 and +14 for UTC 18:00).

Testing the Function #

Let's run this against your original test cases to verify it fixes all the issues:

console.log('At 12am UTC: ' + getValidUTCOffsets(8, 0)); // [8] ✔️
console.log('At 1am UTC: ' + getValidUTCOffsets(8, 1)); // [7] ✔️
console.log('At 8am UTC: ' + getValidUTCOffsets(8, 8)); // [0] ✔️
console.log('At 9am UTC: ' + getValidUTCOffsets(8, 9)); // [-1] ✔️
console.log('At 1pm UTC: ' + getValidUTCOffsets(8, 13)); // [-5] ✔️
console.log('At 3pm UTC: ' + getValidUTCOffsets(8, 15)); // [-7] ✔️
console.log('At 6pm UTC: ' + getValidUTCOffsets(8, 18)); // [-10, 14] ✔️ (matches expected)
console.log('At 8pm UTC: ' + getValidUTCOffsets(8, 20)); // [-12, 12] ✔️ (matches expected)
console.log('At 9pm UTC: ' + getValidUTCOffsets(8, 21)); // [11] ✔️ (invalid -13 is discarded, +11 is kept)
console.log('At 11pm UTC: ' + getValidUTCOffsets(8, 23)); // [9] ✔️ (invalid -15 is discarded, +9 is kept)

All test cases now return the correct valid offsets!

Key Notes #

• The valid UTC offset range (-12 to +14) covers all IANA timezones, including the most extreme ones used in remote islands.
• Using a Set to remove duplicates ensures we don't return the same offset multiple times (this only happens if the base offset is already valid and its ±24 equivalents fall outside the valid range).

内容的提问来源于stack exchange，提问作者HJo