Calendar week view - pressing forwards/backwards buttons sometimes skips a week

[GuntisTreulands]

Describe the bug
While implementing MBCalendarKit in my project and using a week view, I noticed a bug - where sometimes pressing next/previous buttons (to switch to forwards/backwards week), it sometimes skips a week.

To Reproduce
I could easily reproduce this problem using provided demo. In Objective-C Calendar Control -> week view and
Animated Week Transitions.

Steps to reproduce the behaviour skipping a week when pressing next button:

1. Open a week view
2. Click on the LAST date of the visible week (and notice the date)
3. Press next button
4. See that visibly a week has been skipped

Steps to reproduce the behaviour skipping a week when pressing previous button:

1. Open a week view
2. Click on the FIRST date of the visible week (and notice the date)
3. Press previous button
4. See that visibly a week has been skipped

The issue is not reproducable, if, before pressing any button - first/last date is not selected.

Expected behavior
I would expect a week not to be skipped.

Additional context
Tested on iPhone 7, iOS 12

Possible fixes
I found a possible solution that works for me.

For backward button, I added an extra code before this line:

MBCalendarKit/MBCalendarKit/CalendarKit/Core/CKCalendarModel+HeaderViewSupport.m

Line 276 in 9c46b80

date = [self.calendar dateBySubtractingWeeks:1 fromDate:date]; // Add a week

Code:
date = [self.calendar lastDayOfTheWeekUsingReferenceDate:date];
(Basically I set the date as the last day of the visible week, thus making sure that a week will not be skipped)

For forward button, I added an extra code before this line:

MBCalendarKit/MBCalendarKit/CalendarKit/Core/CKCalendarModel+HeaderViewSupport.m

Line 198 in 9c46b80

NSDate *oldDate = date;

Code:
date = [self.calendar firstDayOfTheWeekUsingReferenceDate:date];
(Basically I set the date as the first day of the visible week, thus making sure that a week will not be skipped)