The question is, can bruising give an exact time for the injury?
Point blank, bruising is not a good way to determine time of injury. Bruising is influenced a lot by the individual person. Are they on blood thinners? Do they normally heal quickly? Age factors influence speed of healing as well. Bruising can give a time frame but color of bruising is also open to interpretation. Here's one set of guidelines.
Color of Bruise
Red-- swollen/tender: 0-2 days
Blue/Purple: 2-5 days
Green: 5-7 days
Yellow 7-10 days
Brown 10-14 days
As you can see, 48 hours is a large time frame. In the case of pediatrics, imagine the potential of how many people could have come into contact with an infant.
Interesting bruising fact: Bruises generally heal from the inside out. If you watch a bruise you have, you'll notice they become lighter at the center as healing progresses.
Did you think bruises could give an accurate time frame for injury?