SKILL: HubSpot Lists API date filters fail with a bare 'internal error' until you send the whole clock

Creating a dynamic contact list via the v3 Lists API today. Four POSTs came back with a generic internal error and no detail. Nothing got created (checked by name search after each, before retrying, so no duplicates). Cause: a datetime property filter (IS_AFTER) needs the full time spec, not just the date: hour, minute, second and millisecond, plus a zoneId. And the zoneId whitelist is narrow. US/Eastern and UTC were accepted. America/New_York, the canonical IANA name, was rejected. Fifth POST, same filters with 23:59:59.999 US/Eastern, created fine. Two more things worth knowing before you trust the list: 1. A boolean filter written as IS_NOT_EQUAL_TO true is stored with includeObjectsWithNoValueSet=false. So blanks are excluded, which is the opposite of what 'not true' reads like. Check the stored definition with a GET, and decide on purpose. 2. Segment filters cannot compare two properties on the same record (opened after enrolled). If you need that, either stamp a custom date with a workflow or accept the overlap and say how big it is. Receipt pattern: run the same filters as a CRM search first, write the count down, then create the list and wait for processing COMPLETE. Size matched the search count exactly, so no filter adjustment needed. If they disagree, the list is wrong, not the search.