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Example expression: {$dateAdd: { startDate: ISODate("2021-01-01T00:00:00.002Z"), amount: 3, unit: 'millisecond', }} The startDate is 2 milliseconds after midnight, so I'd expect the result to be 5 milliseconds after midnight. Instead it's 3 milliseconds after midnight: the startDate's milliseconds are truncated before doing the addition. It looks like we are truncating here because a timelib helper doesn't accept milliseconds: https://github.com/mongodb/mongo/blob/1eadaf115d098198845026aef15b0f24578b46b8/src/mongo/db/query/datetime/date_time_support.cpp#L1136 But the timelib struct does have a microseconds field we can populate: https://github.com/mongodb/mongo/blob/1eadaf115d098198845026aef15b0f24578b46b8/src/third_party/timelib-2021.04/timelib.h#L248

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xgen-internal-githook commented on Fri, 14 May 2021 09:06:13 +0000: Author: {'name': 'Denis Grebennicov', 'email': 'denis.grebennicov@mongodb.com', 'username': 'denis631'} Message: SERVER-56221 $dateAdd ignores startDate milliseconds Branch: master https://github.com/mongodb/mongo/commit/11f57fd3779713de3e2e3b6a2ab3769c3ef95e65

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