I usually enter actual time and estimated time in hours or in days, and the tool supports that. The tool doesn't allow defining the rules for translating the units, thought.
For example, if I enter "7 days", the tool might translate that into "One week". Although it doesn't always work out that way, I'd like my Sunday off. So, for me, 7 days mean 7 work days, or one week and two days.
Also, how many work hours make a day, etc.
I find it important that a not-too distant release of the tool supports defining these conversion rules: how many hours in a (work) day, how many (work) days in a week, etc.
