This is a sure conversation topic when talking about high resolution data, including the ones from PMU and COMTRADE data sources.
The PI Time structure is designed based on the standard Unix time_t data type, which was a 32 bit signed integer. That means OSIsoft chose a 16 bit unsigned integer or UInt16 or UShort implementation. This has 65536 possible values ranging from 0 to 65535 with 0 being a subsecond that evenly aligns with a whole second. This approach only requires 2 bytes per timestamp.
Therefore, the PI Server precision is 1/65536.
1 / 65536 second = 0.00000152587890625 second
In North America where the frequency is 60Hz, the time samples is 16 milliseconds, that means the PI timestamp has enough precision to accommodate it. GPS timestamps, which are used by PMUs and PDCs as their time source are accurate to 5 nanoseconds.
Here are some good references when it comes to this topic:
A Detailed Exploration of AFTime Precision
KB: PI Server Storage and precision of Float32 and Float64 data