And they say, "When is this promise, if you should be truthful?" (34:29)

Almuntakhab Fi Tafsir Alquran Alkarim

For this reason they doubt Judgement and they ask: when will that promise of Judgement be fulfilled if indeed you declare the truth

Tafseer Tafheem-ul-Quran by Syed Abu-al-A'la Maududi

(34:29) They ask you: 'When will this promise (of Resurrection) be fulfilled, if what you say is true?' *48

And they say, "When is this promise, if you should be truthful?" meaning

*48) That is, "When will the time come about which you say: `Our Lord will gather u: together, then He will judge between us rightly?' We have been denying you persistently and opposing you openly for so long. Why is not then the judgement being passed against us ?"
 

And they say, "When is this promise, if you should be truthful?" meaning in Urdu

یہ لوگ تم سے کہتے ہیں کہ وہ (قیامت کا) وعدہ کب پورا ہو گا اگر تم سچے ہو؟