I noticed that the duration field is specified as integer in Odoo, but in the Gantt chart it supports floats.
When entering 1.5 week for example in the Gantt chart, the end date is calculated correctly, but when closing the Gantt an reopening it, the duration has become 1 week since only the integer part of the duration is saved in the Odoo model
Support Forum
You can specify the unit of the duration
But I think we should support floats too
https://github.com/bryntum/odoo-support/issues/51