Value Mode in History Properties

Here is a quote from the manual[quote]Analog

The value will be interpolated linearly between the last value and the next value. For example, if the value at Time0 was 1, and the value at Time2 is 3, selecting Time1 will return 2.

In the case above, is Time0 the last OPC value or is it the last value stored in the historian?

It is the OPC value as stored by the historian at Time 0. Don’t be confused by the fact that the historian only logs values “on change” - it also keeps track of how long a tag has stayed at a certain value.