Interval over DIFF nested query

I have a query 

select count() as counter from (select ts, DIFF(variable_value_float) as st_count from table_name where system_uuid = '' and variable_name = '' and ts >= now - 5d and ts <= now - 3d) where st_count = 1 

But when I want to apply an interval at the end as in example below:

select count() as counter from (select ts, DIFF(variable_value_float) as st_count from table_name where system_uuid = '' and variable_name = '' and ts >= now - 5d and ts <= now - 3d) where st_count = 1 interval(1d)

I have an error Window query not supported, since the result of subquery not include valid timestamp column.

Any suggestions?

Also it will be appreciated if you could answer, how I could make a query over multiple variable_name and group by it.