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iphoneobjective-ciosnsdatenscalendar

NSDate - one year ago, dilemma

I'm trying to do something that, for me, is a bit difficult. But I'm sure someone has some insight.

Given a date, say January 17, 2011, I'm trying to figure out the day that corresponds to this date one year ago. So January 17, 2011 is a Monday, and one year ago, this day fell on January 18, 2010 (a Monday as well). It turns out that January 18, 2010 is 354 days before January 17, 2011. I originally thought to just simply subtract 365 days for a non-leap year and 366 days for a leap year, but if you do that in this case, you would get January 17, 2010, which is a Sunday, not a Monday.

So, in Objective-C with NSDate and NSCalendar, how could I implement a function such as:

-(NSDate *)logicalOneYearAgo:(NSDate *)from {
}

In other words, the nth "weekday" of the nth month (where "weekday" is Monday or Tuesday or Wednesday etc)

Solution

  • The answer turned out to be pretty simple. It turned out from trial and error. Here's the answer.

    if the year is a leap year, subtract 365 days.

    if the year is not a leap year, subtract 364 days.