Looking for a programmable watchdog timer, 3 weeks, CubeSat scale

Due to safety concerns about a sub-deployable payload on my CubeSat, NASA is requiring the integration of timer to go in series with the BJT that triggers our payload release mechanism. The timer needs to count down from 3 weeks from when the satellite powers on. Are there any products that fit this application? If not could a series of cascading timers potentially do the trick?