Fixed duration effort off-specifying desired task timescale

My task is fixed duration, effort off but I can’t enter the start and finish dates of this task in this mode, it reverts back to 1 day duration so I have to change the mode, input the information and the change the mode back. Will I have to do this for every task?

All of my tasks are fixed duration effort off. I have a resource who can spend 30% of January working on this task. I want to put in the start and finish dates of this task so I can see the resource on the gannt view with his task showing for the duration of January reflecting the accurate amount of time he will be working throughout the month so it’s not just showing as 1 day duration.

I hoped to put in fixed duration effort off, input the start and end date of this task and put him at 30% but in fixed duration effort off mode, I can’t, the start and finish dates jump to show one day duration so I have to put the task onto another mode, fixed units to specify the dates I want, then add the resource then put the mode back to fixed duration effort off and then input the 30% for January.
Am I doing something wrong or is there an easier way to do this?

Many thanks!