strftime() on Windows returns locale encoding time zone for "%z" even
if MSDN says "%z" is "The offset from UTC in ISO 8601 format; no
characters if time zone is unknown" in MSDN:

https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/strftime-wcsftime-strftime-l-wcsftime-l?view=vs-2019

So we need to convert encoding of string from strftime().