The following is taken from http://muslimkids.multiply.com

Because if you wait until your child turns seven by the calendar of the kuffaar (gregorian), then you are more than two months late in ordering them to pray!  This is because the Gregorian calendar is solar, while the Hijri calendar is lunar and thus is ten days shorter every year than the solar calendar.  I found this very beneficial reminder in my inbox, it was forwarded a bunch of times so I don’t know the site it came from, but brother Moosa Richardson signed off at the end of the statement I pasted below, so I assume it is his words, may Allah reward him.  Regardless of the source, the statement is one of truth and as we know, when the truth comes to us, we take it regardless of the source.

“…Ages of the mashaayikh should be given in hijri years, as we should mention our own ages.  A person who has reached 60 (hijri) years, is 60 years old, not 58 as the non-Muslims would consider him based on their method of keeping track of the years.  This plays an important role in a Muslim’s life, for example the parent is responsible for ordering the child to pray when he reaches seven years (not 7 and a quarter years based on Gregorian calendars).  Thats hijri, not gregorian.  Likewise, 10 years for the other part of the hadeeth.  Likewise, 15 for puberty for those who say that a person is baaligh at 15 if no physical signs have come.  Likewise, when one takes an oath for a month “ash-Shahru hakathaa wa haakathaa, wa haakathaa… ” (“A month is 30 days – or 29).  One who enters into a contract for a “year” – without specifying a year according to the non-Muslims (Gregorian), etc.  This topic was covered somewhat extensively this summer in Toronto.

Perhaps the most important danger in living totally by the Gregorian calender would be: delays in paying zakaat.  For if a person pays every Gregorian year, then he would be 10-11 days late EVERY year.  After 30 years or so he would miss an entire year.  This is a pillar of Islaam.  Actually all four pillars after the shahaadatayn have some reliance on the hijri calender.  So it is important for a Muslim to be thouroughly aqcuainted with it.  May Allaah bless you.

And if you want to convert any significant date (Hijree to gregorian, or vice-versa), the following resource is good:  http://www.rabiah.com/convert/

And Allaah knows best
Moosaa ibn John Richardson

Check this link for hijri date conversion: http://www.islamicfinder.org/dateConversion.php