The Nor gate wont work because the output of the normal clock circuit bit is sometimes high for relatively long periods of time which prevents clocking with the push button. The Nand gate will fail for the same reason only this time when the normal clock circuit bit is low, not high.
That would be true only if the QC output were used directly as the NAND input and is precisely why caps C3 and C4 were introduced in the circuit in post #17, to convert an otherwise prolonged signal to a brief pulse !
rest of the connections are same.
Not so. For the CD4011, pins 1,2,5,6,8,9,12,13 are data inputs; pins 3,4,10,11 are outputs.
For the 7400, pins 1,2,4,5,9,10,12,13 are data inputs; pins 3,6,8,11 are outputs.
one more thing can you help me in understanding the logic behind using nand gate
For the circuit shown:
When pin 1 goes Lo (Mins button pressed) OR pin 2 goes Lo briefly (Seconds count = 60, low pulse duration determined by R4,C4), pins 3,12,13 go Hi so pin 11 goes from Hi to Lo to clock the Mins count. A similar sequence happens for the Hours setting/clocking using the other two gates of the 4011.
MrAl's suggestion has the advantage that caps C3 and C4 would be unnecessary.