Structure.Gantt allows you to define the precision, which is used for scheduling and calculations. You can choose 1 minute, 1 hour or 1 day precision (1 hour is used by default).
The precision defines the minimum meaningful amount of work. The effect of the precision is that all date/time and duration values are rounded according to the precision. For example, with the default 1 hour precision, the duration of 15 minutes will be shown by Gantt as 1 hour, and start date of September 12, 2017 9:45 AM will be shown as September 12, 2017 9:00 AM.
Finish dates and durations are always rounded up.
Gantt would always round the values of Jira fields when doing calculations, but the field value itself is not updated.