An apparatus for selectively isolating a firing head associated with a perforating gun may include an igniter coupled to a firing head, a time delay module coupled to the igniter and generating a pressure pulse after being activated by the igniter, a metering module, and a second firing head. The metering module may be coupled to the time delay module and including a housing having a bore and at least one opening exposed to a wellbore annulus. A piston disposed in the housing bore may have at least one passage. The piston is axially displaced from a first position to a second position by the generated pressure pulse. The second firing head is coupled to the metering module and is in fluid communication with the housing bore. The piston blocks fluid communication from the at least one opening of the housing and the second firing head in a first position and allows fluid communication from the at least one opening of the housing to the second firing head in the second position.