onlne attendance system calculate average time

i am developing online attendance system. i have table t_attendancedetails in sql server.

i have two columns day on attendance and day off attendance..

for example

10-20-2010 09:20:oo ...

i want to calculate average time at which person开发者_如何学C mark his attendance ..and average time at which person mark off his attendance after duty off..

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SELECT
CAST(AVG(CAST( onattendance AS FLOAT )) AS DATETIME),
CAST(AVG(CAST( offattendance AS FLOAT )) AS DATETIME)
FROM t_attendancedetails

This thread may help,

http://www.bennadel.com/blog/175-Ask-Ben-Averaging-Date-Time-Stamps-In-SQL.htm

query taken from there.