This PmTimeDate records when the current overflow high watermark value was established, per its DESCRIPTION, returning an empty string if no overflow watermark has yet been determined. This is the overflow-specific counterpart to the main HighWatermarkTime object, letting an operator pin down exactly which sample interval produced the worst reading during the current run of overflow conditions, as distinct from the worst reading over the whole (longer) Adjustment Interval. For example, if this timestamp keeps advancing every few minutes while OverflowCurrentCount is climbing, that tells you the overflow condition is actively getting worse sample over sample rather than plateauing at one bad reading.