[CentOS] questions relate to "sar"

We have CENTOS 5.3 on DELL server.  I tried to use "sar -b" or "sar -u" and it only show report starting on 12:00 A.M.

my questions are:

1. for "sar -u" or "sar -b" how can I generate two or three days ago report?

2. how to generate daily report from "sa2 " peocess?

Thanks.

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