>>What exactly you're testing right now? Are you testing how long the select statement will take and closing SSMS after each test? Or you're testing the UPDATE statement?
>>
>>I don't expect the good performance when you JOIN based on the time ranges between two tables. It may be better to do an actual loop and using fixed dates intervals.
>>
>>In other words, say, you need to update 10 Clients rows. I expect that even CURSOR based loop for these 10 rows will be quicker than single UPDATE statement.
>
>I do have an interface to do this by code. I was just hoping to be able to do those types of updates from a single command in better timeframes.

For this particular type of the problem I think one by one record approach actually wins over a single update.