Duration fields in CE are tracked at the base level in minutes, then the control that overlays the field on forms and views translates that into larger units (hours, days, weeks, months, etc.) I've run into numerous clients for which this is a particularly difficult thing to get used to, especially in Field Service where the bookings and all of the pieces don't lend themselves well to replacing out of box fields or controls.
I think the first preference should be on training over configuring the application, and get the dispatchers to start thinking in terms of hours instead of days. This will be a much simpler and better solution in the long run, so I'd be remiss to ignore it and jump right to how we can change the application.
Now, with that being said, human behavior is sometimes the most difficult design challenge we have. You could always look at creating a custom field on your Work Order/Task called [Work Days". This could be a decimal field, so they can enter in anything greater than 0 and do half days as well. You then place this field on the form where the standard field was.
You then create a workflow on the task table (real time) that triggers on record create or change to your new custom field. The workflow takes the decimal value entered by the user, and multiplies it by 480 (1 Work Day = 8 Hours x 60 minutes per hour). You then put this value into the Out of Box duration field. Now, you have the system field happy because it is tracking the number of minutes and users are happy because they can enter work days.
1 Work Day = 480 Minutes = 8 CRM Hours
3 Work Days = 1,440 minutes = 24 CRM Hours = 1 CRM Day Duration
If you want to get fancy, you could leave both fields on the form and use workflows to calculate them independently. If someone updates the Work Days field, it updates the CRM Duration field. If they update the CRM duration field, it "translates" it back into Work Days. This is one of those areas where a custom PCF control could probably be created to allow for the user to define what a "Day" is on a duration field, but I have not found one yet.
Let me know if this will work, or if I can provide any other information. Best of luck meeting the requirement!
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