[native] [fix] ENG-949 update timestamp to lowercase

Summary:

update timestamp to match web

web:

Test Plan: open simulator, make sure the timestamp is lowercase

Reviewers: atul, yayabosh, def-au1t, palys-swm, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3576