Assume it is year 2010.
John's 11th birthday was in 2009 but his 12th birthday is still to come this year.
On today's date in 2011 John will still only be 12.
Mary has already had her 12th birthday this year.
Mary had her 11th birthday in 2009 but before that birthday she was still only 10.
On today's date in 2011 Mary will already have had her 13th birthday.
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