spec: create announcements in the past to avoid sub-second precision problems in mysql
spec: create announcements in the past to avoid sub-second precision problems in mysql

refs CNVS-7032

Change-Id: Iccb42f6f2ae2a13f2a3c002305c76ceb405693e0
Reviewed-on: https://gerrit.instructure.com/28803
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Brian Palmer <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@instructure.com>
QA-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

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