Newbie - how to calculate tag data for past 24h

I am very new to PI and am trying to keep up with the demand.   I try looking online but there is too much data for someone new to sort through.

I've been asked to take 4 tags and calculate the data from the past 24 hours and then add them all up.

I have been successful in calculating the total of several tags but not in calculating the tags based on the data from the past 24 hours.  

Any help would be appreciated.

I tried:

Total('SE65BH__0612ENWT','*-24h','*')

TagVal('SE65BH__0612ENWT','*-24h','*')

What am I doing wrong?

Once I get this one tag in the calculation working then I will work to get the others added in.

Thanks