How to change date format to day/month/year in GanttProject software?

Download and install GanttProject from http://www.ganttproject.biz/.

Open GanttProject program.

Go to Menu > Edit > Settings > Application UI.
Use date format: select custom
Custom short date format: type "d/M/yyyy" (for example: 1/7/2123)
Date Format Explanation:
d = single or two digits for day numbers 1-31 in month
M = single or two digits for Month numbers 1-12 in year
mm = lowercase two letters "mm" for minute numbers 00-59 in hour
yyyy = exactly four digits for year

Press OK.