I have not found any way to change this. It does always show in 12-hour format.
The behavior is the same in client and web and changing language settings in Teams does not help.
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This is being discussed as a feature here: https://microsoftteams.uservoice.com/forums/555103-public/suggestions/16935160-use-same-date-and-tim....Feel free to add your vote. I agree on the 24-hour format.
January 23, 2019