An alarm system for indicating the time that has elapsed since intrusion into an enclosed space, as well as a method for installing the alarm system, are disclosed. The alarm system detects an intrusion into the enclosed space, the intrusion thereby causing an indicator light outside the enclosed space to illuminate, thereby indicating time elapsed since intrusion. At least one interior sensor is located within the enclosed space and configured to generate an intrusion time signal in response to movement therein. The indicator light is located within an outer perimeter zone of the enclosed space, and upon receiving an intrusion time signal, the indicator light emits light visible from outside the outer perimeter zone of the enclosed space. The light can indicate time elapsed via a property of the light itself, such as wavelength, intensity and/or focus, and/or via a light display producing readable output of time elapsed, for example.