Project managers don’t normally want to manage the "time of day" within project tasks. If a task is going to take three hours within a single day, we don't worry about the specific time that those three hours of work will take place. Sometime during the eight hour day is just fine.
The problem with task settings in MS Project is that the default settings always force task work values based on duration. This “trains” a lot of project managers into making those simple few hour tasks into partial day durations to avoid messing around with all kinds of other calculations (what percent of the 8 hour day is 3 hours?). Dealing with partial days in a work breakdown structure later on during schedule changes is no fun!
The Alternative to the Default MS Project Task Calculations
You can change the default settings so that tasks are calculated as fixed units, and are NOT effort driven. In addition, using the "Work" column next to the "Duration" column, you can tab across your task entries with manual entries for duration and work values. You can let the software figure out the percent allocation of the resource, and stick to entering partial day tasks with one day duration values and a few hours of work.
Not only does this help in the avoidance of partial day duration values on one day tasks, but it also simplifies the process of determining resource allocation for many longer duration tasks. It’s often easiest to estimate how many hours a resource needs to complete a task, and what time frame they will need in order to perform those hours of work.