This read-only DisplayString is a timestamp marking when the current 15-minute interval bucket began accumulating statistics. An admin uses it to anchor the numeric interval bucket (from cmDSLIntervalNumber) to an actual wall-clock time when correlating a DSL error spike with other logs or customer-reported outage times. For example, seeing this timestamp read a time three hours ago confirms that bucket's error counters describe activity from that exact three-hour-old window.