I've been using Alimentor for almost a year now. It is invaluable for not only tracking actual time spent for each parent with our child, but also for documenting incidents of special interest and things one might need down the road in custody disputes.
For example, if my son has a new bruise at school when I pick him up, I snap a photo with few notes for future reference.
If he has a bed wetting accident, I log that too. If he tells me of something questionable that happens when he's not with me, I make a note of that as well. You get the idea.
This is all very valuable data over the long run. If his mother asks about any bed wetting, I can quickly look it up and tell her when he's had accidents at my house. If she asks about a rash when she picks him up, I can reference my diary memo in Alimentor to give her the details.
So, I use Alimentor for time tracking, special incidents of note, and anything else that I may need in any potential disputes in the future. I don't have to rely on memory or what I think has been happening.