PI Performance Equations do not provide event-weighted standard deviations for a single tag over an interval. They provide time-weighted standard deviation, or event-weighted for a list of explicit arguments as opposed to for a tag and a time interval.
You can instead use a Totalizer and the StdDeviation function. It provides both time-weighted and event-weighted standard deviations. Please refer to the PI Server Applications User guide for more information. Also, since this is not exactly a programming question, I would suggest you seek help with our regular Tech Support folks in case you hit further issues on how Totalizers work. They are in much better position for these types of questions.