You can easily convert all 3 of your columns into proper Date/Time columns for "Date hour preparation" and "date hour use" and into a duration column for the column called "Difference".
I will start with the column "Difference"
1. Right click on the column header for column "Difference" and choose "Column Info". You will see the following screen
![change.PNG change.PNG](https://community.jmp.com/t5/image/serverpage/image-id/9237iEB94A1FFC6F91481/image-size/large?v=v2&px=999)
2. Change the Data Type to Numeric. (A New format field will be displayed)
3. Change the Modeling Type to Coninuous
4. Change the Format to
Duration==>hr:m
The window will now look like
![Change2.PNG Change2.PNG](https://community.jmp.com/t5/image/serverpage/image-id/9238iBDC551996361A7F4/image-size/large?v=v2&px=999)
5. Click on the "Apply" button and the Difference Column will be changed to Numeric/Continuous
You can repeat the above steps for the 2 date/time columns, but instead of selecting a Duration format, select
Time==>d/m/y h:m
![change3.PNG change3.PNG](https://community.jmp.com/t5/image/serverpage/image-id/9239i528360A92FABD476/image-size/large?v=v2&px=999)
Which results in a data table looking like
![change4.PNG change4.PNG](https://community.jmp.com/t5/image/serverpage/image-id/9241iA46A24897B86810D/image-size/large?v=v2&px=999)
Jim