change to os_time to prevent unmanagable clock skew

[bunnylushington]

Changed System.system_time(:second) to System.os_time(:second) per the recommendation in #147.

Note that the skew, at least on my system, was pretty extreme:

{:system_time, {:ok, ~U[2023-04-28 11:22:57Z]}}
{:os_time, ~U[2023-04-28 18:38:17.936225Z]}

[wojtekmach]

Thank you!