The normal working hours of the Calendar are (8:00-12:00 13:00-17:00). There is an abnormal working day record in the Calendar, and its working hours are (8:00-16:00).When you add a normal Task to ExtGantt, change its start date to an exceptional working day, and then change its end date to the date of any other normal working day.This will result in the selection of a normal workday whose end date is at 16:00 instead of 17:00. Causes an error in the duration value calculation. The ExtGantt version is 6.1.13
Actually no need I've managed to reproduce it. Thing is date field applies time of the last provided value in case it uses a format w/o time info.
I'll make a feature request to snap to the chosen date working time end in such cases.
Here it is: https://app.assembla.com/spaces/bryntum/tickets/9660-end-date-field-should-snap-to-the-working-time-end/details