وَإِذْ وَاعَدْنَا مُوسَىٰ أَرْبَعِينَ لَيْلَةً ثُمَّ اتَّخَذْتُمُ الْعِجْلَ مِن بَعْدِهِ وَأَنتُمْ ظَالِمُونَ
Wa iz waa'adnaa Moosaaa arba'eena laylatan summat takhaztumal 'ijla mim ba'dihi wa antum zaalimoon
And remember when we appointed forty nights for Musa, and in his absence you took up the calf — and you were wrongdoers.
Wāʿadnā is a verb of mutual appointment, so the forty nights are a meeting arranged rather than an absence imposed. The tradition counts them as thirty nights of Dhu'l-Qaʿda followed by ten of Dhu'l-Hijja, and it noted that the lapse falls inside the appointed time rather than outside it. What the passage measures is what happens in the gap left when the one who leads steps away.