I recall first time when i used calendar I could not understand what i need to do to change selected time period in the calendar.
I spent a couple of minutes (minutes namely) to understand that i need to click on date after changing hours or minutes.

I thought - why there is no Ok button? Many of similar calendars have Ok button and it's intuitively understandable that i need to click Ok after changing something manually.

All this time every time when i used calendar (for change hours or minutes) i feel the same - where is damn Ok button?
And now i have no more patience to be silent

Additionally is not intuitively understandable what i need to click for close calendar when i open it accidentally. Ok button can be very useful for such cases.

I think (i hope) i'm not alone and i propose to add this button. See attached picture.