Let H be a self-adjoint operator on \(L^2(X)\) and generate a bounded holomorphic semigroup of angle \(\frac{\pi }{2}\) . In this paper, we study the following questions. First, if \(e^{-zH}\) is an integral operator with integral kernel K(z, x, y), then, under what circumstances, K(z, x, y) has a boundary limit and the limit becomes the integral kernel of \(e^{-itH}\) . On the other hand, if \(e^{-itH}\) is an integral operator with integral kernel K(it, x, y), whether \( e^{-zH} \) is also an integral operator and how to construct the integral kernel from K(it, x, y), if it exists. Finally, we apply the results to study the joint continuity of the integral kernel of \(e^{-itH}\) where \(H= -\Delta + V\) and \(V\in L^{2}(\mathbb {R}^\nu )\) such that the Fourier transform of V has compact support.