得到一周的起始时间

+ (NSDate *)getWeekFromDate:(NSDate *)date
{
    if (date == nil) {
        date = [NSDate date];
    }
    double interval = 0;
    NSDate *beginDate = nil;
    NSDate *endDate = nil;
    
    NSCalendar *calendar = [NSCalendar currentCalendar];
    [calendar setFirstWeekday:2];//设定周一为周首日
    BOOL ok = [calendar rangeOfUnit:NSWeekCalendarUnit startDate:&beginDate interval:&interval forDate:date];
    //分别修改为 NSDayCalendarUnit NSWeekCalendarUnit NSYearCalendarUnit
    if (ok) {
        endDate = [beginDate dateByAddingTimeInterval:interval-1];
    }
    
    return beginDate;
}