Here Allah tells His Messenger that he cannot know when the Hour will come, and if people ask him about that, He instructs him to refer the matter to Allah, may He be exalted, as Allah says in Surat Al-A`raf, even though that was revealed in Makkah and this Surah was revealed in Al-Madinah. Allah continues to tell him to refer this matter to the One Who knows about it, but He tells him that it is at hand, as He says:

﴿وَمَا يُدْرِيكَ لَعَلَّ السَّاعَةَ تَكُونُ قَرِيباً﴾

(What do you know It may be that the Hour is near!) This is like the Ayat:

﴿اقْتَرَبَتِ السَّاعَةُ وَانشَقَّ الْقَمَرُ ﴾

(The Hour has drawn near, and the moon has been cleft asunder.) ﴿54:1﴾

﴿اقْتَرَبَ لِلنَّاسِ حِسَـبُهُمْ وَهُمْ فِى غَفْلَةٍ مُّعْرِضُونَ ﴾

(Draws near for mankind their reckoning, while they turn away in heedlessness.) ﴿21:1﴾

﴿أَتَى أَمْرُ اللَّهِ فَلاَ تَسْتَعْجِلُوهُ﴾

(The Event (the Hour) ordained by Allah will come to pass, so seek not to hasten it) ﴿16:1﴾.