## 1 Introduction  
Apertures in conducting planes are frequently encountered in the form of windows, cracks around doors, coupling slots in microwave devices, or imperfect seams between two metallic plates (e.g., ([1], Chapter 4) and ([2], Sections 6.7 and 7.9)). In order to quantify both intentional and undesired effects of such apertures, their EM scattering is a major concern in applied electromagnetics.  
Wavefield penetration through an aperture in a thin screen is a classical problem of wavefield physics, solutions of which can be traced back to the work of [PERSON] (see [3] and ([4], SS 6.1)). His solution applying to the plane-wave diffraction by a relatively small (with respect to the wavelength) aperture was later extended by [PERSON] and [PERSON] [5; 6; 7]. Without restricting himself to the plane-wave excitation, [PERSON] [8] demonstrated that the EM scattering by a small aperture can be, in certain circumstances (see ([5], SS 9)), attributed to the action of equivalent electric and dipole moments [9; 10]. Whenever the maximum linear dimension of the aperture is not sufficiently small with respect to the operating wavelength, however, the approximate aperture-diffraction models due to [PERSON] and [PERSON] are no longer applicable and one has to resort to more general formulations. A way out of the difficulty has been offered by [PERSON] and [PERSON] through their variational formulation [11; 12], thus extending the steady-state solution to a wide range of frequencies.  
The available rigorous solutions of the aperture diffraction problem were derived under the assumption of sinusoidally in time-varying wavefields. However, owing to the widespread use of communication and radar systems relying on the transmission, detection, and subsequent interpretation of digital signals, the steady-state assumption may no longer be computationally efficient and/or physically legitimate. Therefore, in the present work, we remove the restriction and solve the diffraction wavefield problem in its (original) space-time domain. The vast majority of previous works aiming at TD solutions rely more or less on the pertaining steady-state results--exact, purely TD analytical solutions of the aperture diffraction are lacking in the literature on the subject. Indeed, the pertinent solutions can be, broadly speaking, divided into two categories. The first line of reasoning employes purely numerically approaches such as the inverse FFT algorithm [13; 14] or the finite-difference TD technique [15; 16]. The second approach yields TD approximate field expressions based on the use of [PERSON]'s model [17] or [PERSON]'s approximation ([18], Section 5.5.1). To deal with the issue, the classic CdH technique [19] (see also [20; 21; 22; 23], for example) and the TD EM reciprocity theorem of the time-convolution type ([24], Section 28.2) (see also ([25], Section 5.2), ([26], Section 1.4.1), and [27]) are in the present work combined to introduce a novel, rigorous TD integral-equation approach to analyzing the pulsed EM scattering by a relatively small rectangular aperture in a PEC screen. The new solution strategy, to be referred to as the CdH-MoM, has been previously applied to TD performance studies of cylindrical and planar antennas [28] and, more recently, to the TD EM scattering analysis of transmission lines in the presence of thin sheets [29] and EM metasurfaces [30; 31].