时间类型字段更新

哪位大侠帮忙看看，
表jobhis

sp_help jobhis

jobid char 8
dt_plan datetime 8
dt_exec datetime 8

select * from jobhis

jobid dt_plan dt_exec
20110103 2011-01-03 00:00:00.0 2011-01-04 09:17:29.0
20110105 2011-01-05 00:00:00.0 2011-01-07 09:04:49.0
20110110 2011-01-10 00:00:00.0 2011-01-20 08:50:54.0
20110112 2011-01-12 00:00:00.0 2011-01-20 08:50:15.0
...
...

我想更新dt_exec字段为dt_plan相同日期，时分秒保持不变
更新后预计结果为
jobid dt_plan dt_exec
20110103 2011-01-03 00:00:00.0 2011-01-03 09:17:29.0
20110105 2011-01-05 00:00:00.0 2011-01-05 09:04:49.0
20110110 2011-01-10 00:00:00.0 2011-01-10 08:50:54.0
20110112 2011-01-12 00:00:00.0 2011-01-12 08:50:15.0
...
...

或者更新dt_exec为dt_plan相同日期内的任一时间点也可以，不知道有没有这个随机函数。