Tweak convertUptime() to not rely on external date

[yubiuser]

What does this PR aim to accomplish?:

For converting the uptime in seconds to days, hours, minutes we used date but used some functions of it are not available on all versions of date (see #426).

This PR gets rid of date and uses arithmetics to do the conversion instead.

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[yubiuser]

Confirmed working in #426 (comment)