إِنَّ هَؤُلَاءِ مُتَبَّرٌ مَا هُمْ فِيهِ وَبَاطِلٌ مَا كَانُوا يَعْمَلُونَ ﴿۱۳۹﴾ قَالَ أَغَيْرَ اللَّهِ أَبْغِيكُمْ إِلَهًا وَهُوَ فَضَّلَكُمْ عَلَى الْعَالَمِينَ ﴿۱۴۰﴾ وَإِذْ أَنْجَيْنَاكُمْ مِنْ آلِ فِرْعَوْنَ يَسُومُونَكُمْ سُوءَ الْعَذَابِ يُقَتِّلُونَ أَبْنَاءَكُمْ وَيَسْتَحْيُونَ نِسَاءَكُمْ وَفِي ذَلِكُمْ بَلَاءٌ مِنْ رَبِّكُمْ عَظِيمٌ ﴿۱۴۱﴾ وَوَاعَدْنَا مُوسَى ثَلَاثِينَ لَيْلَةً وَأَتْمَمْنَاهَا بِعَشْرٍ فَتَمَّ مِيقَاتُ رَبِّهِ أَرْبَعِينَ لَيْلَةً وَقَالَ مُوسَى لِأَخِيهِ هَارُونَ اخْلُفْنِي فِي قَوْمِي وَأَصْلِحْ وَلَا تَتَّبِعْ سَبِيلَ الْمُفْسِدِينَ ﴿۱۴۲﴾

﴾139﴿ Innaa haaa'ulaaa'i mutabbarum maa hum feehi wa baatilum maa kaanoo ya'maloon
﴾140﴿ Qaala a-ghairal laahi abgheekum ilaahanw wa Huwa faddalakum 'alal 'aalameen
﴾141﴿ Wa iz anjainaakum min Aali Fir'awna yasoomoo nakum sooo'al 'azaab, yuqattiloona abnaaa'akum wa yastahyoona nisaaa'akum; wa fee zaalikum balaaa'um mir Rabbikum 'azeem
﴾142﴿ Wa waa'adnaa Moosaa salaaseena lailatanw wa at mamnaahaa bi'ashrim fatamma meeqaatu Rabbihee arba'eena lailah; wa qaala Moosaa liakheehi Haaroonakh lufnee fee qawmee wa aslih wa laa tattabi' sabeelal mufsideen

﴾139﴿ Indeed these people will perish, what they are engaged in worshiping, and the deeds they do are invalid
﴾140﴿ Musa (peace be upon him) said, Should I ask someone other than Allah, the Most High, to be a deity for you, and He has given you superiority over the people of this time
﴾141﴿ And when we saved you from Pharaoh's people, they brought you a bad punishment They killed your sons and left your women alive indecently, and in this blessing was great (trial) from your Lord
﴾142﴿ And we made a promise with Musa (peace be upon him) of thirty nights, and we completed it in another ten, so the appointment set by God for forty nights was completed, And he said, Musa, peace be upon him, his brother Harun, peace be upon him Be my successor in my people and correct them And do not follow the way of corrupters

[139] The purpose of the first sentence is to say that these objects of worship are perishable and will be destroyed by the monotheists, and from Allah the Exalted's side, their worship is also not permissible in any form.
The purpose of the second sentence is to emphasize that these polytheistic acts are useless—they can neither bring benefit nor repel harm.
[140] This is a warning from Musa (peace be upon him) to his people. First, he mentioned that these objects of worship are not worthy of being worshipped. Now, he states that Allah the Exalted alone is deserving of worship with true monotheism, due to a reason that is a virtue. The path of this virtue is mentioned in verse 20 of Surah Al-Ma'idah.
[141] In this statement lies the reason for the virtue of the Children of Israel and a reminder of Allah the Exalted’s blessings. Similar verses are mentioned in verse 49 of Surah Al-Baqarah and verse 6 of Surah Ibrahim.
[142] From here begins the fifth section, up to verse 154. In this portion, the first appointed meeting (miqat) is mentioned—for the giving of the Book and the appointment of Harun (peace be upon him) as deputy—and it includes the account of Musa (peace be upon him) requesting to see Allah the Exalted, followed by the incident of the Children of Israel falling into shirk by worshipping the calf, and the anger of Musa (peace be upon him) towards them.
These forty nights included the month of Dhu al-Qa'dah and ten days of Dhu al-Hijjah. According to narrations, after one month, Musa (peace be upon him) used a miswak (tooth-stick), and because of that, Allah the Exalted added ten more days. This is why the phrase "(بِعَشْرٍ)" (with ten) is mentioned separately. During these ten days, the Torah was given to him.
Question: Why was the phrase "(فَتَمَّ مِيقَاتُ رَبِّهِ)" (so the appointed time of his Lord was completed) necessary?
It is to remove a misconception—so that no one thinks the ten days were part of the thirty, but rather, they were added after the thirty.
The difference between "miqat" and "waqt" is that waqt refers to time in a general sense, while miqat refers to a time specifically appointed for a certain task.