Swift中DateFormatter的HH格式未输出24小时制时间问题求助

This is a classic edge case tied to iOS 11.x's date formatter behavior—you’re absolutely right to suspect the older system version here. Here’s what’s happening and how to fix it:

The Root Cause #

On iOS 11.2 and some earlier minor versions, even when you set the time zone to UTC, the DateFormatter (or NSDateFormatter for Objective-C) might still respect the device’s regional time format preferences (12-hour vs 24-hour) if you don’t explicitly lock down the locale. This inconsistency doesn’t appear on newer iOS versions or even same-version devices like your iPhone X, likely because those devices have 24-hour format enabled in system settings, while your cousin’s iPhone doesn’t.

The Solution: Force POSIX Locale #

To guarantee 24-hour format output regardless of device settings or iOS version, you need to set the formatter’s locale to en_POSIX. This locale is standardized and ignores system-level regional preferences, ensuring your format string is interpreted exactly as written.

Swift Example:

let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd HH:mm:ss" // "HH" = 24-hour format
dateFormatter.timeZone = TimeZone(identifier: "UTC")
// Critical line to bypass system locale preferences
dateFormatter.locale = Locale(identifier: "en_POSIX")

let currentUTCTime = dateFormatter.string(from: Date())
print(currentUTCTime) // Will always output 24-hour format

Objective-C Example:

NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];
[dateFormatter setTimeZone:[NSTimeZone timeZoneWithName:@"UTC"]];
// Lock locale to POSIX to avoid regional overrides
[dateFormatter setLocale:[[NSLocale alloc] initWithLocaleIdentifier:@"en_POSIX"]];

NSString *currentUTCTime = [dateFormatter stringFromDate:[NSDate date]];
NSLog(@"%@", currentUTCTime);

Key Notes #

• Always use HH (uppercase) for 24-hour format in your date string—hh (lowercase) is for 12-hour format.
• The en_POSIX locale is safe to use globally; it won’t affect the language of your date output unless you’re using localized date components (which you aren’t here, since you’re using a fixed format string).

内容的提问来源于stack exchange，提问作者Dipen Shah