Well, the Hijri date 21 Muharram 1187 corresponds to the Gregorian date Tuesday, 13 April 1773. This date lies in the first month of the Hijri year 1187 AH, which is Muharram of 1187 AH. Both this Hijri and Gregorian date occur on the single day that is Tuesday without any doubt. The Arabic date 1187/01/21 is calculated using the Umm Al-Qura calendar and the sighting of the moon. One thing to remember is that this Arabic date may occur on different Gregorian date depending upon the region and country and obviously the moon.
If you are still not sure about the date then you can use our Arabic date converter.
The following is the conversion of the Gregorian date 13 April 1773 to its equivalent Arabic date.
The following is the conversion of the Arabic date 21 Muharram 1187 AH to its equivalent Gregorian date.
لِّيَجْزِىَ ٱللَّهُ ٱلصَّٰدِقِينَ بِصِدْقِهِمْ وَيُعَذِّبَ ٱلْمُنَٰفِقِينَ إِن شَآءَ أَوْ يَتُوبَ عَلَيْهِمْ ۚ إِنَّ ٱللَّهَ كَانَ غَفُورًۭا رَّحِيمًۭا
That Allah may reward the true men for their truth, and punish the hypocrites if He will, or relent toward them (if He will). Lo! Allah is Forgiving, Merciful.
Surah Al-Ahzaab(33:24)