The clock operates over spans of seconds to minutes after an RFID chip is charged up from an RFID reader or other ambient radio-wave energy. As a result, even after the radio signal is removed, the clock endows the RFID chip with the ability to know when its security keys may be in danger.
via Could an SRAM Hourglass Save RFID Chips Just in Time? – IEEE Spectrum.
Having a clock can be very useful in defending against brute-force attacks that may try to guess the chip’s passwords hundreds or thousands of times per second. A TARDIS-enabled chip—requiring no new hardware and representing fewer than 50 lines of additional code—would receive a power-up from, say, a nearby RFID reader. Instead of wiping the SRAM clean, the device would first read off the state of the SRAM, which would be partially decayed from the last time the chip was powered up. Comparing the percentage of decayed bits to a precompiled table would enable TARDIS to read off the time elapsed since the previous power-up.