some pretty basic sql-features : count( distinct col)
and some basic date-timetamp manipulations
could lead to this following statement :
select count( distinct employ_number) from employ_table where log_timestamp between
timestamp(current date - 24 days,'00:00') and (timestamp(current date - 17 days,'00:00') - 1 microsecond )
I can explain it to you, but i can not understand it for you.