iphone之日期与时间（转载）

@interface NSDate (Helper)
- (NSUInteger)daysAgo;
- (NSUInteger)daysAgoAgainstMidnight;
- (NSString *)stringDaysAgo;
- (NSString *)stringDaysAgoAgainstMidnight:(BOOL)flag;
- (NSUInteger)weekday;
+ (NSDate *)dateFromString:(NSString *)string;
+ (NSDate *)dateFromString:(NSString *)string withFormat:(NSString *)format;
+ (NSString *)stringFromDate:(NSDate *)date withFormat:(NSString *)string;
+ (NSString *)stringFromDate:(NSDate *)date;
+ (NSString *)stringForDisplayFromDate:(NSDate *)date;
+ (NSString *)stringForDisplayFromDate:(NSDate *)date prefixed:(BOOL)prefixed;
- (NSString *)string;
- (NSString *)stringWithFormat:(NSString *)format;
- (NSString *)stringWithDateStyle:(NSDateFormatterStyle)dateStyle timeStyle:(NSDateFormatterStyle)timeStyle;
- (NSDate *)beginningOfWeek;
- (NSDate *)beginningOfDay;
- (NSDate *)endOfWeek;
+ (NSString *)dateFormatString;
+ (NSString *)timeFormatString;
+ (NSString *)timestampFormatString;
+ (NSString *)dbFormatString;
@end
#import "NSDate+Helper.h"



@implementation NSDate (Helper)



- (NSUInteger)daysAgo {

NSCalendar *calendar = [NSCalendar currentCalendar];

NSDateComponents *components = [calendar components:(NSDayCalendarUnit)

fromDate:self

toDate:[NSDatedate]

options:0];

return [components day];

}



- (NSUInteger)daysAgoAgainstMidnight {

// get a midnight version of ourself:

NSDateFormatter *mdf = [[NSDateFormatter alloc]init];

[mdf setDateFormat:@"yyyy-MM-dd"];

NSDate *midnight = [mdf dateFromString:[mdf stringFromDate:self]];

[mdf release];

return (int)[midnight timeIntervalSinceNow] / (60*60*24) *-1;

}



- (NSString *)stringDaysAgo {

return[self stringDaysAgoAgainstMidnight:YES];

}



- (NSString *)stringDaysAgoAgainstMidnight:(BOOL)flag {

NSUInteger daysAgo = (flag) ? [selfdaysAgoAgainstMidnight] : [selfdaysAgo];

NSString *text = nil;

switch (daysAgo) {

case0:

text = @"Today";

break;

case1:

text = @"Yesterday";

break;

default:

text = [NSStringstringWithFormat:@"%d days ago", daysAgo];

}

return text;

}



- (NSUInteger)weekday {

NSCalendar *calendar = [NSCalendar currentCalendar];

NSDateComponents *weekdayComponents = [calendar components:(NSWeekdayCalendarUnit) fromDate:self];

return [weekdayComponents weekday];

}



+ (NSDate *)dateFromString:(NSString *)string {

return[NSDate dateFromString:string withFormat:[NSDate dbFormatString]];

}



+ (NSDate *)dateFromString:(NSString *)string withFormat:(NSString *)format {

NSDateFormatter *inputFormatter = [[NSDateFormatter alloc]init];

[inputFormatter setDateFormat:format];

NSDate *date = [inputFormatter dateFromString:string];

[inputFormatter release];

return date;

}



+ (NSString *)stringFromDate:(NSDate *)date withFormat:(NSString *)format {

return [date stringWithFormat:format];

}



+ (NSString *)stringFromDate:(NSDate *)date {

return [date string];

}



+ (NSString *)stringForDisplayFromDate:(NSDate *)date prefixed:(BOOL)prefixed {

NSDate *today = [NSDatedate];

NSCalendar *calendar = [NSCalendar currentCalendar];

NSDateComponents*offsetComponents = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit)

fromDate:today];

NSDate *midnight = [calendar dateFromComponents:offsetComponents];

NSDateFormatter *displayFormatter = [[NSDateFormatter alloc]init];

NSString *displayString = nil;

// comparing against midnight

if ([date compare:midnight] == NSOrderedDescending) {

if (prefixed) {

[displayFormatter setDateFormat:@"'at' h:mm a"]; // at 11:30 am

} else {

[displayFormatter setDateFormat:@"h:mm a"]; // 11:30 am

}

} else {

// check if date is within last 7 days

NSDateComponents *componentsToSubtract = [[NSDateComponentsalloc] init];

[componentsToSubtract setDay:-7];

NSDate *lastweek = [calendar dateByAddingComponents:componentsToSubtract toDate:today options:0];

[componentsToSubtract release];

if ([date compare:lastweek] == NSOrderedDescending) {

[displayFormatter setDateFormat:@"EEEE"]; // Tuesday

} else {

// check if same calendar year

NSInteger thisYear = [offsetComponents year];

NSDateComponents*dateComponents = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit)

fromDate:date];

NSInteger thatYear = [dateComponents year];

if (thatYear >= thisYear) {

[displayFormatter setDateFormat:@"MMM d"];

} else {

[displayFormatter setDateFormat:@"MMM d, yyyy"];

}

}

if (prefixed) {

NSString *dateFormat = [displayFormatter dateFormat];

NSString *prefix = @"'on' ";

[displayFormatter setDateFormat:[prefix stringByAppendingString:dateFormat]];

}

}

// use display formatter to return formatted date string

displayString = [displayFormatter stringFromDate:date];

[displayFormatter release];

return displayString;

}
+ (NSString *)stringForDisplayFromDate:(NSDate *)date {

return[self stringForDisplayFromDate:date prefixed:NO];

}
- (NSString *)stringWithFormat:(NSString *)format {

NSDateFormatter *outputFormatter = [[NSDateFormatter alloc]init];

[outputFormatter setDateFormat:format];

NSString *timestamp_str = [outputFormatter stringFromDate:self];

[outputFormatter release];

return timestamp_str;

}
- (NSString *)string {

return[self stringWithFormat:[NSDatedbFormatString]];

}
- (NSString *)stringWithDateStyle:(NSDateFormatterStyle)dateStyle timeStyle:(NSDateFormatterStyle)timeStyle {

NSDateFormatter *outputFormatter = [[NSDateFormatter alloc]init];

[outputFormatter setDateStyle:dateStyle];

[outputFormatter setTimeStyle:timeStyle];

NSString *outputString = [outputFormatter stringFromDate:self];

[outputFormatter release];

return outputString;

}
- (NSDate *)beginningOfWeek {

// largely borrowed from "Date and Time Programming Guide for Cocoa"

// we'll use the default calendar and hope for the best

NSCalendar *calendar = [NSCalendar currentCalendar];

NSDate *beginningOfWeek = nil;

BOOLok = [calendar rangeOfUnit:NSWeekCalendarUnit startDate:&beginningOfWeek

interval:NULL forDate:self];
if (ok) {
return beginningOfWeek;
}
// couldn't calc via range, so try to grab Sunday, assuming gregorian style
// Get the weekday component of the current date
NSDateComponents*weekdayComponents = [calendar components:NSWeekdayCalendarUnit fromDate:self];
NSDateComponents *componentsToSubtract = [[NSDateComponentsalloc] init];

[componentsToSubtract setDay: 0 - ([weekdayComponents weekday] - 1)];
beginningOfWeek = nil;
beginningOfWeek = [calendar dateByAddingComponents:componentsToSubtract toDate:selfoptions:0];
[componentsToSubtract release];
//normalize to midnight, extract the year, month, and day components and create a new date from those components.
NSDateComponents*components = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit)
fromDate:beginningOfWeek];

return [calendar dateFromComponents:components];
}
- (NSDate *)beginningOfDay {
NSCalendar *calendar = [NSCalendar currentCalendar];
// Get the weekday component of the current date
NSDateComponents*components = [calendar components:(NSYearCalendarUnit | NSMonthCalendarUnit | NSDayCalendarUnit)
fromDate:self];
return [calendar dateFromComponents:components];
}
- (NSDate *)endOfWeek {
NSCalendar *calendar = [NSCalendar currentCalendar];
// Get the weekday component of the current date
NSDateComponents*weekdayComponents = [calendar components:NSWeekdayCalendarUnit fromDate:self];
NSDateComponents *componentsToAdd = [[NSDateComponents alloc]init];
// to get the end of week for a particular date, add (7 - weekday) days
[componentsToAdd setDay:(7 - [weekdayComponents weekday])];
NSDate *endOfWeek = [calendar dateByAddingComponents:componentsToAdd toDate:selfoptions:0];
[componentsToAdd release];
return endOfWeek;
}
+ (NSString *)dateFormatString {
return @"yyyy-MM-dd";
}
+ (NSString *)timeFormatString {
return @"HH:mm:ss";
}
+ (NSString *)timestampFormatString {
return @"yyyy-MM-dd HH:mm:ss";
}
// preserving for compatibility
+ (NSString *)dbFormatString {
return[NSDate timestampFormatString];
}

@end