ENOfficial
It was narrated that Abu Ma`shar said: `Uthman (رضي الله عنه) was killed on the eighteenth of Dhul-Hijjah 35 AH, His caliphate lasted twelve years less twelve days.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا إِسْحَاقُ بْنُ عِيسَى بْنِ الطَّبَّاعِ، عَنْ أَبِي مَعْشَرٍ، قَالَ وَقُتِلَ عُثْمَانُ رَضِيَ اللَّهُ عَنْهُ يَوْمَ الْجُمُعَةِ لِثَمَانِ عَشْرَةَ مَضَتْ مِنْ ذِي الْحِجَّةِ سَنَةَ خَمْسٍ وَثَلَاثِينَ وَكَانَتْ خِلَافَتُهُ ثِنْتَيْ عَشْرَةَ سَنَةً إِلَّا اثْنَيْ عَشَرَ يَوْمًا.
sunnah.com