What's the best way to store time I need to store "time" in a content type. The time is actually duration (how long it took people to perform various tasks), with the time varying between a few minutes to several days. 

Is there anything already out there (Date doesn't seem to cover it)?

If not, what's the best way to create this? 

Ideally I'd like dropdowns for days, hours, minutes, seconds, tenths.

It seems to make sense to store a computed value in the database (total seconds, rather than save everything in five separate fields. Are there any good tutorials on doing this?

I assume I then use a field formatter to break out 300.3 seconds into 6:00.3 or whatever format I want?
Sounds like you're looking for something like Time Period.

That one stores the interval in seconds, so if you need smaller values you will probably have to write your own field widget/formatter but the module might still serve as a base.. or maybe you could extend it to cover this use case as well and e.g. store microseconds.