Handling period durations of days with `T` suffix (#80)

* Improved test

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Added more fix cases

Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>

* Use assert signature

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Signed-off-by: Arnau Mora <arnyminer.z@gmail.com>
Co-authored-by: Ricki Hirner <hirner@bitfire.at>