UTC24小时格式转本地12小时格式及逆向转换的OC代码问题

Hey there! Let's work through your date conversion needs step by step, fixing potential issues in the provided code and sharing reliable implementations.

1. Convert UTC 24-hour String to Local 12-hour Format #

Your goal is to turn a UTC string like 20-Nov-2019 21:09 into a local 12-hour format (e.g., 20-Nov-2019 04:09 PM depending on your time zone).

Issue with Your Current Code #

The code you shared skips a critical first step: parsing the input UTC string into an NSDate object. You tried to reconfigure the same date formatter and call stringFromDate:date, but didn't define how date was created from your input string. Here's the corrected approach:

Correct Implementation #

// Step 1: Parse the UTC string into an NSDate object
NSString *utcInputString = @"20-Nov-2019 21:09";
NSDateFormatter *utcParser = [[NSDateFormatter alloc] init];
utcParser.dateFormat = @"dd-MMM-yyyy HH:mm";
utcParser.timeZone = [NSTimeZone timeZoneWithAbbreviation:@"UTC"];
// Use en_US_POSIX to avoid locale-specific parsing errors
utcParser.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
NSDate *utcDate = [utcParser dateFromString:utcInputString];

// Step 2: Convert the NSDate to a local 12-hour format string
NSDateFormatter *localFormatter = [[NSDateFormatter alloc] init];
localFormatter.dateFormat = @"dd-MMM-yyyy hh:mm a";
localFormatter.timeZone = [NSTimeZone localTimeZone];
// Use current locale for natural AM/PM formatting
localFormatter.locale = [NSLocale currentLocale];
NSString *local12HourString = [localFormatter stringFromDate:utcDate];

NSLog(@"Local 12-hour result: %@", local12HourString);
// Example output (for UTC-5 time zone): 20-Nov-2019 04:09 PM

2. Convert Local 12-hour Format to UTC 24-hour Format #

You provided two methods for this conversion, but we can refine them to be more reliable and avoid unnecessary manual time interval calculations.

Improved Implementation #

// Convert a local date string (supports both 24-hour and 12-hour formats) to UTC 24-hour string
- (NSString *)convertLocalDateStringToUTC:(NSString *)localDateString {
    // Step 1: Parse the local string into an NSDate
    NSDateFormatter *localParser = [[NSDateFormatter alloc] init];
    localParser.timeZone = [NSTimeZone localTimeZone];
    localParser.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
    
    // Try 24-hour format first
    localParser.dateFormat = @"dd-MMM-yyyy HH:mm";
    NSDate *localDate = [localParser dateFromString:localDateString];
    
    // Fall back to 12-hour format if needed
    if (!localDate) {
        localParser.dateFormat = @"dd-MMM-yyyy hh:mm a";
        localDate = [localParser dateFromString:localDateString];
    }
    
    // Step 2: Convert to UTC 24-hour string
    if (localDate) {
        NSDateFormatter *utcFormatter = [[NSDateFormatter alloc] init];
        utcFormatter.dateFormat = @"dd-MMM-yyyy HH:mm";
        utcFormatter.timeZone = [NSTimeZone timeZoneWithName:@"UTC"];
        utcFormatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
        return [utcFormatter stringFromDate:localDate];
    }
    
    return nil; // Return nil if parsing fails
}

// Alternative: Convert an existing NSDate (local time) directly to UTC string
- (NSString *)convertLocalDateToUTCString:(NSDate *)localDate {
    NSDateFormatter *utcFormatter = [[NSDateFormatter alloc] init];
    utcFormatter.dateFormat = @"dd-MMM-yyyy HH:mm";
    utcFormatter.timeZone = [NSTimeZone timeZoneWithName:@"UTC"];
    utcFormatter.locale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
    return [utcFormatter stringFromDate:localDate];
}

Key Best Practices #

• Always set locale to en_US_POSIX when parsing fixed-format date strings. This prevents issues with device-specific locale settings (like different month abbreviations or 12/24-hour preferences).
• Explicitly define the timeZone for both parsing and formatting to ensure you're working with the correct time context.
• Remember: NSDate objects represent absolute time points—time zones only matter when converting between dates and human-readable strings.

内容的提问来源于stack exchange，提问作者Naresh