This PmTimeDate records when the current underflow high watermark value was established, per its DESCRIPTION, returning an empty string if no underflow watermark has yet been determined. Despite the 'high' in its name (inherited from the same naming pattern used for the overflow and main watermark objects), this timestamp actually marks when the worst (lowest) reading of the current underflow run occurred, so treat 'high watermark' here as 'the most extreme recorded reading', not literally the highest number. For example, this timestamp updating every sample interval while UnderflowCurrentCount keeps climbing shows the underflow condition setting a new, lower low each time rather than holding steady at one bad reading.