Hello; is there a function or a calculation I can use to Convert between Hijra to Gregorian Date in my FM database? 

I worked on this at one time, but found out there isn't a consensus on what the correct algorithm (or even the correct result) is and put it aside.

I would suggest you find an API that conforms to your method and use that - see, for example:
https://aladhan.com/islamic-calendar-api

 

Hello and thank you.  I did see an API from this link you provided " http://api.aladhan.com/v1/hToG?date=14-02-1436" Now how I can use it in my FM Apps.?

 

HTTP/1.1 200 OK { { code: 200, status: "OK", data: { hijri: { date: "14-02-1436", format: "DD-MM-YYYY", day: "14", month: { number: 2, en: "Ṣafar", ar: "صَفَر" }, year: "1436", designation: { abbreviated: "AH", expanded: "Anno Hegirae" } }, gregorian: { date: "07-12-2014", format: "DD-MM-YYYY", day: "07", month: { number: 12, en: "December" }, year: "2014", designation: { abbreviated: "AD", expanded: "Anno Domini" } } } } }

See the attached demo.

 

IslamicCalendarAPI.fmp12

Excellent and Thanks, what will happen if user does not have Internet Connection when converting dates?

6 minutes ago, Ismail AlAbdulmohsen said:

if user does not have Internet Connection

It won't work.