may I ask and need your help also. I am using a full calendar plugin latest version of v3.8.0. I create an event which is the start date is 2018-01-16 23:59:00 and the end date is 2018-01-17 08:00:00 (11:59pm - 8:00am) . The event is plotted only on 1 box which is the day of 16, but when is set it at 2018-01-16 23:59:00 and the end date is 2018-01-17 09:00:00 (11:59pm - 9:00am) the result is set 2 box from 16-17 day.I'm having a hard time on setting this time format for this plugin.
I assume you're talking about the "month" type of view, because this is the only situation where this behaviour occurs. It doesn't make any logical sense in the other view types.
Anyway, this behaviour is controlled by the nextDayThreshold setting. When an event's end time spans into another day, this option controls minimum end time it must have in order for it to be rendered onto that day. The default value is 9am
This can be useful if you have an event which ends in the early hours (e.g. a party) but you don't want people to confused into thinking it takes place the next evening, or runs over into the normal working day or something.
In your case, if you wanted your event to appear, you could set the threshold earlier, for example 7am, in order to change the behaviour:
nextDayThreshold: '07:00:00'
Here are two working demos showing you the difference when you change the threshold:
Default behaviour: http://jsfiddle.net/sbxpv25p/93/
With nextDayThreshold set to 7am: http://jsfiddle.net/sbxpv25p/94/
See https://fullcalendar.io/docs/event_rendering/nextDayThreshold/ for more detail about this option.