Threshold Set Expires Date

Some regulatory standards have explicit sunset dates — for example, interim action levels that apply only until a final cleanup goal is established. The expiresDate on a ThresholdSet records this end date, signaling to Piper's exceedance detection that results sampled after expiresDate should be evaluated against a different, current set rather than the expired one. Compliance staff can use expiresDate to plan ahead: when a ThresholdSet is approaching its expiration, they know to create and assign the successor set before the current one lapses to avoid gaps in exceedance coverage.