Get first day in a month from NSCalendar

问题

I have this subset of a method that needs to get day one of the current month.

NSDate *today = [NSDate date];  // returns correctly 28 february 2013
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
NSDateComponents *components = [[NSDateComponents alloc] init];
components.day = 1;
NSDate *dayOneInCurrentMonth = [gregorian dateByAddingComponents:components toDate:today options:0];

dayOneInCurrentMonth then prints out 2013-03-01 09:53:49 +0000, the first day of the next month.

How do I get day one of the current month?

回答1:

Your logic is wrong: Instead of setting the date's day to 1, you're adding a day to the current date.

Try something like that:

NSDate *today = [NSDate date];
NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSDateComponents *components = [gregorian components:(NSEraCalendarUnit | NSYearCalendarUnit | NSMonthCalendarUnit) fromDate:today];
components.day = 1;

NSDate *dayOneInCurrentMonth = [gregorian dateFromComponents:components];

回答2:

You want [NSCalender dateFromComponents:] instead:

NSDate *dayOneInCurrentMonth = [gregorian dateFromComponents:components];

回答3:

Easy way of getting to the 1st from a given date:

NSDate *first = [gregorian dateBySettingUnit:NSCalendarUnitDay value:1 ofDate:date options:0]; 

回答4:

Easy Way to get first and last date of previous month is :

NSCalendar *gregorian = [[NSCalendar alloc] initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
NSDateComponents *comp = [gregorian components:(NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay ) fromDate:[NSDate date]];

//TO GET PREVIOUS MONTH LAST DAY
[comp setMonth:[comp month]];
[comp setDay:1];
NSDate *tDateMonth = [gregorian dateFromComponents:comp];
NSLog(@"LAST DAY OF PREVIOUS MONTH ;  %@", tDateMonth);

//TO GET PREVIOUS MONTH FIRST DAY
[comp setMonth:[comp month]-1];
[comp setDay:3];
NSDate *td = [gregorian dateFromComponents:comp];
NSLog(@"FIRST DAY OF PREVIOUS MONTH ;  %@", td);

Hope this helps

回答5:

By creating new variable is expensive on memory usage, it's better using date formatter to get the first day of the current month

today = [NSDate date];

firstDayDateFormatter = [[NSDateFormatter alloc] init];
[firstDayDateFormatter setDateFormat:@"01-MM-yyyy"];
dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:@"dd-MM-yyyy"];
[dateFormatter setLocale:[NSLocale localeWithLocaleIdentifier:@"en_US"]];
firstDayOfTheMonth = [mdfDateFormat dateFromString:[firstDayDateFormatter stringFromDate:today]];

回答6:

NSDate *startDate = nil;
[[NSCalendar currentCalendar] rangeOfUnit:NSCalendarUnitMonth startDate:&startDate interval:NULL forDate:date];

return startDate;

来源：https://stackoverflow.com/questions/15132547/get-first-day-in-a-month-from-nscalendar