imagine this is 1960-70.....a country has nuclear plants already (for peaceful purposes) now if they get the technology of gas centrifuges what things they are going to need for building two small nukes within a year? and if they want to detonate this like a time bomb, is it possible?
I don't know about the first part of your question, I think gas centrifuge specifications and capabilities may be classified which would make a reasonably exact estimate quite hard. The Fat Man nuclear bomb (20kt-yield, implosion design) required 6.2kg of plutonium (or, if the country is unwilling to take the extra step of converting its uranium to plutonium in nuclear reactors, about 13kg of highly enriched uranium would give off a similar yield).
Nuclear bombs can be designed to detonate based on any number of conditions (specific atmospheric pressure, sudden acceleration, manual command, etc.) and using a timer to trigger the bomb would be very simple.
The timer would send an electric signal to the control circuits for the bomb, which would then trigger fast, synchronized pulses to special detonators (often exploding-bridgewire detonators). The detonators would initiate a layer of conventional high explosives, which would send a shockwave to compress a core of uranium or plutonium until it reaches the critical density necessary to sustain a chain fission reaction, at which point it explodes violently, releasing a lot more energy (several orders of magnitude) than given off by the initial (conventional HE) explosion.