如何……

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"yyyy"];

//Optionally for time zone conversions
[formatter setTimeZone:[NSTimeZone timeZoneWithName:@"..."]];

NSString *stringFromDate = [formatter stringFromDate:myNSDateInstance];

//unless ARC is active
[formatter release];

Swift 4.2:

func stringFromDate(_ date: Date) -> String {
    let formatter = DateFormatter()
    formatter.dateFormat = "dd MMM yyyy HH:mm" //yyyy
    return formatter.string(from: date)
}

如何……

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
[formatter setDateFormat:@"yyyy"];

//Optionally for time zone conversions
[formatter setTimeZone:[NSTimeZone timeZoneWithName:@"..."]];

NSString *stringFromDate = [formatter stringFromDate:myNSDateInstance];

//unless ARC is active
[formatter release];

Swift 4.2:

func stringFromDate(_ date: Date) -> String {
    let formatter = DateFormatter()
    formatter.dateFormat = "dd MMM yyyy HH:mm" //yyyy
    return formatter.string(from: date)
}

迅速:

var formatter = NSDateFormatter()
formatter.dateFormat = "yyyy"
var dateString = formatter.stringFromDate(YourNSDateInstanceHERE)

NSCalendar *calendar = [NSCalendar currentCalendar];
NSDateComponents *components = [calendar components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay) fromDate:myNSDateInstance];
NSInteger year = [components year];
// NSInteger month = [components month];
NSString *yearStr = [NSString stringWithFormat:@"%ld", year];

简单的方法，使用c#风格的方式将日期转换为字符串。

用法:

let a = time.asString()
// 1990-03-25


let b = time.asString("MM ∕ dd ∕ yyyy, hh꞉mm a")
// 03 / 25 / 1990, 10:33 PM

扩展:

extension Date {
    func asString(_ template: String? = nil) -> String {
        if let template = template {
            let df = DateFormatter.with(template: template)
            
            return df.string(from: self)
        }
        else {
            return globalDateFormatter.string(from: self)
        }
    }
}

// Here you can set default template for DateFormatter
public let globalDateFormatter: DateFormatter = DateFormatter.with(template: "y-M-d")

public extension DateFormatter {
    static func with(template: String ) -> DateFormatter {
        let df = DateFormatter()
        df.dateFormat = template
        return df
    }
}

如果你在Mac OS X上，你可以这样写:

NSString* s = [[NSDate date] descriptionWithCalendarFormat:@"%Y_%m_%d_%H_%M_%S" timeZone:nil locale:nil];

但是这在iOS上是不可用的。