I have heard and read many times that reading Dua Ahad for forty mornings after Namaz e Fajr will for sure be in serving Imam Mehdi army. I tried many times but one or other day I missed but still I would recite in zuhr. My question is do reading dua ahad even on zuhr or any other time will help joining Imam army please?

The dua can be said anytime but to attain what the narration mentioned it says the mornings.