Template:Countdown a

{{subst:subst}}#ifexpr:{{{2}}}>0|{{subst:subst}}countdown|1={{{1}}}|2={{subst:#expr:{{{2}}}-1}}}}
{{{2}}} {{{1}}}}}

The wikitext without "<includeonly></includeonly>":

{{subst:subst}}#ifexpr:{{{2}}}>0|
{{subst:subst}}countdown|{{{1}}}|
{{subst:#expr:{{{2}}}-1}}}}<br>{{{2}}} {{{1}}}}}

Also without "subst:", for further readibility:

{{subst}}#ifexpr:{{{2}}}>0|{{subst}}countdown|{{{1}}}|{{#expr:{{{2}}}-1}}}}<br>{{{2}}} {{{1}}}}}

Substituting this template produces one or more new substitution template calls, allowing recursive change of a page, see meta:Help:Recursive conversion of wikitext.