~yerinalexey reported this issue. In fact, POSIX does not define --date or an equivalent, so I'm a bit surprised it works at all. GNU date supports relative items, while BusyBox only supports a more limited set of formats:

Recognized TIME formats:

hh:mm[:ss]
[YYYY.]MM.DD-hh:mm[:ss]
YYYY-MM-DD hh:mm[:ss]
[[[[[YY]YY]MM]DD]hh]mm[.ss]

I'm not sure of any way to easily fix this unless there is some other way to use relative dates in BusyBox. Especially since we use another GNUism, --date, throughout the script, effort might be better spent rewriting omnavi in a different language than bash+coreutils.