Simple method for calculating a date that was one week, one month, 6 months, or a year ago.

Date subtraction

// DateFilter is just an enum for some preset dates to filter on.
- (NSDate *) dateForFilter:(DateFilter)filterType
{
    NSDate *today = [NSDate date];
    NSCalendar *cal = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    
    NSDateComponents *dateComp = [[NSDateComponents alloc] init];
    
    switch (filterType) {
        case DateFilterWeek:
            dateComp.week = -1;
            break;
        case DateFilterMonth:
            dateComp.month = -1;
            break;
        case DateFilter6Month:
            dateComp.month = -6;
            break;
        case DateFilterYear:
            dateComp.year = -1;
            break;
        default:
            break;
    }
    
    NSDate *pastDate = [cal dateByAddingComponents:dateComp toDate:today options:0];
    return pastDate;
}