Spec timestamp parsing for feeds without offsets

Your analytics warehouse takes events from 4 partner feeds. Some timestamps carry an offset, like 2026-03-08T01:30:00-06:00, and some do not, like 2026-03-08 01:30:00. Last month one partner's traffic showed up on a dashboard as 2 humps, hours apart, and the cause was a parsing bug. An agent will rewrite the Python function that turns every feed's timestamp into one stored instant.

Write the spec you would hand the agent.

What you know:

• feed A sends ISO 8601 with Z or an offset
• feed B sends YYYY-MM-DD HH:MM:SS in the partner's local time, and the partner is in America/Chicago
• feed C sends MM/DD/YYYY HH:MM with no offset, and nobody has asked the partner which timezone it is
• feed D sends Unix epoch seconds
• the ingester runs on servers in 2 regions, set to different system timezones

Then, in a few sentences: why did the old code give different results on different servers?