poezio/mam: Remove superfluous time format conversions and fix parameter type

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>
7 jobs for master in 2 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
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allowed to fail
pylint-plugins

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pytest-3.5

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pytest-3.6

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passed #22635
pytest-latest

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passed #22632
security-check

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  Build
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www
build-doc

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build-ubuntu

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