>I have seen some code which is as follows
>
>select names
>set order to tag datex
>set key to dtot({})
>
>The file is ordered by date time
>
>Does the set key set a filter to today's date?
>
>If not what does it do?

You should finaly learn how to use this forum, in special the pre tags. Is it realy that tricky to copy a couple of lines out of a reply?

How to test?

?dtot({})
?dtot({})=DATETIME()
?dtot(DATE())=DATETIME()
?dtot(DATE())

DATETIME is a second, you compare the first second of today with any other second of today. This will not match. Or only by a chance of 1/86400.

The riddle might be solved with

HELP SET KEY

Pro tip:

set key to RANGE dtot(DATE()),dtot(DATE()+1)-1
*IOW SET KEY to a range between the first second of today and a second before the first second of tomorow, aka the last second of today.

Alles klar?

:D