Automatic End Date not working correctly

Scheduler Javascript
#1

Automatic end time is not working correctly. I have the following scheduler configuration:

scheduler.config.limit_time_select = true;

scheduler.config.first_hour = 7;

scheduler.config.last_hour = 20;

scheduler.config.auto_end_date = true;

scheduler.config.event_duration = 120;

scheduler.config.time_step = 15;

scheduler.config.details_on_create = true;

My time range is between 7 am and 8 pm. Time marks between 7 am and 6 pm relate to an end time, which is 2 hours ahead => so far so good!. If i choose i.e. 6pm + 15min, the end time is switched to an empty field, which is not correct. In my opinion it should get the value 8pm instead of 8pm + 15min.

Is it possible to manually set the end time in relation to the choosen starttime?

#2

i forgot to say, that i refer to the time selector of lightbox.

On the screenshot i have selected 7pm as start time, but the end time field does change to blank.