Specify base as utc or localtime to let the RTC start at the current UTC or local time,
respectively. localtime is required for correct date in MS-DOS or Windows.
To start at a specific point in time, provide datetime in the format 2006-06-17T16:01:21 or 2006-06-17. The default base is UTC.
ref https://www.qemu.org/docs/master/about/removed-features.html#localtime-removed-in-3-1
qemu 3.1でオプションが無くなった
https://www.qemu.org/docs/master/system/qemu-manpage.html
MS-DOSで必要な機能だったようだ