date addition unit tests

- (void)testDateAdditions {
	int daysAgoReversed = [[NSDate dateAtMidnightWithDaysAgo:900] numDaysAgo];
	STAssertTrue(daysAgoReversed == 900, @"date methods should be reversible, but 900 != %d", daysAgoReversed);

	NSDate *tomorrow = [[NSDate date] dateByAddingDays:1];
	int tomorrowDaysAgo = [tomorrow numDaysAgo];
	STAssertTrue(tomorrowDaysAgo == -1, @"tomorrow should be negative one day ago, but it was %d, "
				 "and we think tomorrow is %f seconds from 1970", tomorrowDaysAgo, [tomorrow timeIntervalSince1970]);
	
	NSDate *yesterday = [[NSDate date] dateByAddingDays:-1];
	int yesterdayDaysAgo = [yesterday numDaysAgo];
	STAssertTrue(yesterdayDaysAgo == 1, @"yesterday should be one day ago, but it was %d, and we "
				 "think yesterday is %f seconds from 1970", yesterdayDaysAgo, [yesterday timeIntervalSince1970]);
	
	
	NSDate *today = [NSDate date];
	int todayDaysAgo = [today numDaysAgo];
	STAssertTrue(0 == todayDaysAgo, @"today should be 0 days ago, but it was %d", todayDaysAgo);
	
	NSDate *now = [NSDate date];
	NSDate *nowWithNoDaysAdded = [now dateByAddingDays:0];
	
	STAssertTrue(fabs([now timeIntervalSince1970] - [nowWithNoDaysAdded timeIntervalSince1970]) < 1, 
				 @"adding no days should not change , but fabs(%f - %f) >= 1", [now timeIntervalSince1970],
				 [nowWithNoDaysAdded timeIntervalSince1970]);
}