"5a" works as "5am" not because "a" is considered as "am" , but because by default the "5:00" is "5 AM".
The same way "5z" will be considered as 5 AM, and "5sometext" will be considered as "5 AM"
To get PM you have to provide correct text that to be parsed in PM, otherwise the parsing will consider it as 5:00 which is AM.
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