Crystal Reports Retrieve Min of Time

Message

     Dear all,

I have a report with Grouping by DocNo, under each grouping, there is a list of data, one of the data is Time. Eg:

DocNo: 0001
Time
08:03
08:05

I would like to retrieve the min of time, means 08:03 to show at the group header.

I use Running Total Field, 
Field to summarize: Time
Evaluate: On Change of Group - DocNo
Reset: On Change of Group - DocNo

But I unable get the valid minimum time at group header.. any error on my analysis?

Thanks.