function processLine(line, operation, timezone) {
    if (!line.trim() || line.trim().startsWith('#')) {
        return line;
    }

    try {
        let date;
        const trimmedLine = line.trim();

        switch (operation) {
            case 'unix':
                // Input is seconds
                date = new Date(Number(trimmedLine) * 1000);
                break;
            case 'unix_micro':
                // Input is milliseconds
                date = new Date(Number(trimmedLine));
                break;
            case 'to_utc':
            case 'from_utc':
                // Input is ISO 8601
                date = new Date(trimmedLine);
                break;
        }

        if (isNaN(date.getTime())) {
            return 'Invalid Date';
        }

        if (operation === 'to_utc') {
            let dateToConvert = date;
            if (timezone === 'utc' && !trimmedLine.toUpperCase().endsWith('Z') && !trimmedLine.match(/[+-]\d{2}:?\d{2}$/)) {
                dateToConvert = new Date(trimmedLine + 'Z');
            }
            return dateToConvert.toISOString();
        }

        if (operation === 'from_utc') {
            let dateFromUtc;
            if (!trimmedLine.toUpperCase().endsWith('Z') && !trimmedLine.match(/[+-]\d{2}:?\d{2}$/)) {
                dateFromUtc = new Date(trimmedLine + 'Z');
            } else {
                dateFromUtc = new Date(trimmedLine);
            }

            if (timezone === 'utc') {
                return dateFromUtc.toISOString();
            } else {
                // Local
                const offset = dateFromUtc.getTimezoneOffset();
                const localDate = new Date(dateFromUtc.getTime() - (offset * 60 * 1000));
                return localDate.toISOString().slice(0, -1);
            }
        }

        // For Unix conversions:
        if (timezone === 'utc') {
            return date.toISOString();
        } else {
            // Local
            const offset = date.getTimezoneOffset();
            const localDate = new Date(date.getTime() - (offset * 60 * 1000));
            return localDate.toISOString().slice(0, -1);
        }

    } catch (e) {
        return 'Error';
    }
}

// Tests
console.log("Running tests...");

// 1. Unix to UTC
const t1 = processLine('1672531200', 'unix', 'utc');
console.log(`1. Unix to UTC: ${t1} (Expected: 2023-01-01T00:00:00.000Z)`);
if (t1 !== '2023-01-01T00:00:00.000Z') console.error("FAIL");

// 2. Unix Micro to UTC
const t2 = processLine('1672531200000', 'unix_micro', 'utc');
console.log(`2. Unix Micro to UTC: ${t2} (Expected: 2023-01-01T00:00:00.000Z)`);
if (t2 !== '2023-01-01T00:00:00.000Z') console.error("FAIL");

// 3. To UTC (from Local input)
// Note: Node.js server might be in UTC or Local. 
// If I run this on a machine, "Local" depends on system time.
// However, the logic for 'to_utc' with 'utc' timezone forces Z.
const t3 = processLine('2023-01-01T00:00:00', 'to_utc', 'utc');
console.log(`3. To UTC (input no offset, selected UTC): ${t3} (Expected: 2023-01-01T00:00:00.000Z)`);
if (t3 !== '2023-01-01T00:00:00.000Z') console.error("FAIL");

// 4. From UTC (to UTC)
const t4 = processLine('2023-01-01T00:00:00', 'from_utc', 'utc');
console.log(`4. From UTC (input no offset, selected UTC): ${t4} (Expected: 2023-01-01T00:00:00.000Z)`);
if (t4 !== '2023-01-01T00:00:00.000Z') console.error("FAIL");

// 5. Comment
const t5 = processLine('# comment', 'unix', 'utc');
console.log(`5. Comment: ${t5} (Expected: # comment)`);
if (t5 !== '# comment') console.error("FAIL");

console.log("Tests finished.");