Using relative times can be tricky depending on when things become evaluated. Are you trying to get the TimeStamp or the value? To get the TimeStamp, please use e.g. PrevEvent() At what frequency is the tag serviced? As said, before relative times can be tricky because you never know exactly when the evaluation happens. PrevVal('BREAKER','*') can refer the current snapshot or the previous (archive) event. You can use a little trick to move the equation to the safe side e.g. PrevVal('BREAKER','*+1s') should always return the snapshot value if the sample frequency is > 1s.