With `star`, your command line looks:

    touch -t201802280000 ref
    star -c -f file.tar -find . -newermm ref

BTW: -newermt may be a nice future enhancement for `libfind`.

If you like an end date as well:

    touch -t201802280000 start
    touch -t201802282359 end
    star -c -f file.tar -V pat='start!end' -find . -newermm start ! -newermm end

Note that you either have to omit one second or you may get two days if the years was a leap year.

Since 2018 isn't a leap year, you may use `touch -t201803010000 end`.