~yerinalexey reported this issue. It is similar to #1.
%s is a format specifier for "seconds since the Epoch (1970-01-01 00:00 UTC)". It is implemented by GNU date but not specified by POSIX.
In the discussion on IRC, ~ecs mentioned this lovely gem as a potential alternative:
$(($(date -u +"((%Y-1600)*365+(%Y-1601)/4-(%Y-1601)/100+(%Y-1601)/400+1%j-1000-135140)*86400+(1%H-100)*3600+(1%M-100)*60+(1%S-100)")))
It is an improved snippet from this guide.
I do not yet know how should be addressed, but probably in the same way as #1.
No,
+%s
is valid in busybox
Oh, sorry. Wires crossed between the two issues.