This app performs various operations for the Hijri and Gregorian calendars.
- The (Hijri-Gregorian Calendar) page displays the two calendars in one calendar, in addition to displaying the current time and the date of the Hijri and Gregorian day at the top of the page.
- The (Date Converter) page converts the date entered between the two dates.
- The (Age Calculation) page calculates the entered age in days, weeks, months, and years in the Hijri and Gregorian dates.
- The (difference between two dates) page calculates the difference in days, weeks, months, and years in Hijri or Gregorian depending on the two dates entered.

In this app, the following things have been relied upon:

First: Hijri Dates:
We calculated the Hijri date in two ways
1: The Umm al-Qura calendar was relied on for the dates between 1/1/1356 and 29/12/1500 corresponding to the Gregorian dates between 14/3/1937 and 15/11/2077
2: A special calculation method was used on other dates based on the basis that the Hijri year consists of 12 months, and the days in each month are as follows:
Month 1 (Muharram) has 30 days
Month 2 (Safar) has 29 days
Month 3 (Rabi` al-Awwal) has 30 days
Month 4 (Rabi` al-Thani) has 29 days
Month 5 (Jumada al-Awwal) has 30 days
Month 6 (Jumada al-Akhira) has 29 days
Month 7 (Rajab) includes 30 days
Month 8 (Shaban) has 29 days
Month 9 (Ramadan) has 30 days
Month 10 (Shawwal) has 29 days
Month 11 (Dhu al-Qi'dah) has 30 days
Month 12 (Dhu al-Hijjah) has 29 days
A day has been added to the last month (Dhu al-Hijjah) in leap years to become 30 days.
The leap years in the second method were calculated as follows: In every 30 years there are 11 leap years (years 2, 5, 7, 10, 13, 16, 18, 21, 24, 26, and 29)
In this method, the first day of the first Hijri year was adopted on 1/1/1 corresponding to Friday on the Gregorian date 16/7/622.
Note: There is always a possibility of error in calculating the Hijri date, with an increase or decrease of two days at worst,
due to the difference in calculating the beginning of the monthly Hijri entry between countries.

Second: Gregorian dates:
We also adopted two methods of calculating the Gregorian dates:
1: The Julian date from 1/1/1 to 4/10/1582.
2: The Gregorian date and starts on 15/10/1582.
There are 10 days deleted in this method, extending from 5/10/1582 to 14/10/1582, which were equalized as follows:
5/10/1582 = 15/10/1582
6/10/1582 = 16/10/1582
7/10/1582 = 17/10/1582
8/10/1582 = 18/10/1582
9/10/1582 = 19/10/1582
10/10/1582 = 20/10/1582
11/10/1582 = 21/10/1582
12/10/1582 = 22/10/1582
13/10/1582 = 23/10/1582
14/10/1582 = 24/10/1582
The difference between the two methods is in the calculation of a leap year.
In Julian dates, a leap year is always counted every 4 years, adding one day to the second month (February) to become 29 days.
In Gregorian history, a leap year is also calculated every 4 years by adding one day to February.
But the difference in Gregorian history, if the year is divisible by 100, then it is not a leap unless it is divisible by 400,
for example, the year 1700 is divisible by 4 and by 100, but it is not divisible by 400 so it is not a leap year.
But the year 2000 is considered as a leap year.