Average of dates and times if it meets a specific criteria

Hi guys,

i need some help averaging the time between dates and times.

i need it to average the dates and times if it meets a certain condition for instance if the name is "Adams".

The best i can come up with is {=AVERAGE(IF(B3:B27="Adams",D3:D27))} but that gives me the average date between the values not the hours.

e.g. if the range is:

12/07/2010 06:00
13/07/2010 06:00
14/07/2010 06:00

the answer once it's formatted correctly is 13/07/2010 06:00 but i need it to say the average hours between so the answer should be 24 hours and not the middle date.

Hope that all makes sense and someone can help

Regards,
Harry