The Prophet ﷺ said, "We are an illiterate nation; we neither write, nor know accounts. The month is like this and this, i.e. sometimes of 29 days and sometimes of thirty days."  

عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ إِنَّا أُمَّةٌ أُمِّيَّةٌ لاَ نَكْتُبُ وَلاَ نَحْسُبُ الشَّهْرُ هَكَذَا وَهَكَذَا يَعْنِي مَرَّةً تِسْعَةً وَعِشْرِينَ وَمَرَّةً ثَلاَثِينَ