We have already dealt with a similar issue here. They might appear unrelated but they are actually caused by the same underlying problem. Your phone is "old" period. You can temporarily solve this problem by turning your phone's clock backwards. Sometimes this works but sometimes it doesn't. See http://answers.techzim.co.zw/23291/install-whatsapp-nokia-asha-mine-says-certificate-not-phone
The short answer is find an Android or if you have the bucks an iPhone. I would advise you to buy an original android phone at the very least: Huawei, Samsung and Lenovo have budget Android devices instead of buying those Gulf Complex phones or buying one outside of Econet. These phones are usually crappy and have rubbish specs which will make you rue the decision sooner or later.
Also do not buy your phone from those second hand sellers in town. I have friends who spend nights in jail after they were tracked and caught. The police in this country arrest and then investigate.