I'm new to the list, so somebody might already answered these questions,
but anyway...
1. Singular/Plural
Japanese language don't distinguish singular nor plural.
Just fill in both "minute" and "minutes" same strings and you're ok.
2. Placement or order of the numbers
Also note that, in the example cases you've shown below, both English
and Japanese words "follows" after numerical number. This is not always
true for other languages...
Also note that, if you want to show date in shorter form in year, month,
day, we usually prefer to use:
Y/M/D
notation and not
D/M/Y
nor
M/D/Y
3. Selection of the expressions.
I think the example you've shown below is just fine.
If you want to use some I18N modules with the computer language you're
using, there are another (long) story on this.
My best bet might be use of strftime (like) function with "C" locale or
without locale support. For example,
1 minute ago => 1分前
%M minutes ago => %M分前
# note that strftime's %M is two-digit output. I prefer trimming '0'.
shigeya