Convert datetime to HNI

Summary: fixes #3863

Reviewed By: @JoelMarcey

Differential Revision: D1638773

Signature: t1:1638773:1417566513:96549af96db9215b7e53c1ffe13ea080641503b5
Convert math to HNI

Summary: fixes #1684

Convert ext/standard/math to HNI

Reviewed By: @ptarjan, @JoelMarcey

Differential Revision: D1646888

Signature: t1:1646888:1414613111:207e8c5ce9c600857a98461a4848aec6bf739647

Pulled By: @nigelchanyk