Adds beginningOfWeek and beginningOfDay instance methods

NSDate+Helper.h

@@ -23,4 +23,7 @@
 + (NSString *)stringForDisplayFromDate:(NSDate *)date;
 + (NSString *)stringForDisplayFromDate:(NSDate *)date prefixed:(BOOL)prefixed;
 
+- (NSDate *)beginningOfWeek;
+- (NSDate *)beginningOfDay;
+
 @end

NSDate+Helper.m

@@ -146,4 +146,35 @@ + (NSString *)stringForDisplayFromDate:(NSDate *)date {
 	return [self stringForDisplayFromDate:date prefixed:NO];
 }
 
+- (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];
+	// Get the weekday component of the current date
+	NSDateComponents *weekdayComponents = [calendar components:NSWeekdayCalendarUnit fromDate:self];
+
+	/*
+	 Create a date components to represent the number of days to subtract from the current date.
+	 The weekday value for Sunday in the Gregorian calendar is 1, so subtract 1 from the number of days to subtract from the date in question.  (If today's Sunday, subtract 0 days.)
+	 */
+	NSDateComponents *componentsToSubtract = [[NSDateComponents alloc] init];
+	[componentsToSubtract setDay: 0 - ([weekdayComponents weekday] - 1)];
+	NSDate *beginningOfWeek = [calendar dateByAddingComponents:componentsToSubtract toDate:self options: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];
+}
+
 @end