Here is the writeup on TimeStampSource for history tags.
When a Tag executes, there are two possible timestamps that can be observed: the time associated with the data, and the time that the tag was evaluated. The first case is generally only interesting when the value is provided by an OPC server. In most cases, the time provided by OPC, which in Ignition is referred to as the “Value” time, will be very close to the system time. Some servers, however, either due to their location or how they function (history playback, for example), will provide times that are very different than the current time.
It is generally desirable to store the System time, as it is the time that the value was actually observed by the system, and it creates a uniform timeframe for all realtime data. However, in the later case, it is necessary to store the time provided by the OPC server. Using the Value timestamp source has several consequences: the system is no longer able to validate the tag quality against the scan class’ execution, and tag value interpolation will behave differently.
The validation of the scan class execution is generally not a concern when recording historical playback data. Interpolation only occurs when the value mode is Analog, and when there is not a value for every time window. Using System time, the value is only interpolated during the last “scan class execution window”, that is, one scan class timeframe before the next value. Using Value time, however, the value is interpolated for the entire time between two data points.
docs.inductiveautomation.com/di … Tags+Intro