You cannot just change all the cars in a country, never mind multiple countries at the same time, that quickly.
It takes about 3 years to develop a new car from design start to production, and that's assuming the key technologies already work. Then the car is in production for at least 5 years, usually more, and once the car has been bought it will be in used for between 12 and 20 years. So a car in regular use today could have been built in 1999, started production in 1993, and therefore development started in 1990.
The owner of that 1999 car might be able to afford to replace it with a 2005 car, but not a brand-new 2019 car, and definitely not a brand-new 2019 electric car that costs 50% more than a similar-sized petrol car due to battery costs.
Switching to electric cars is more difficult than just replacing old cars with new. There isn't yet the battery build capacity to build enough batteries to power the electric cars currently being built (hence the order books for some of Hyundai and Kia's electric cars have been closed until next year). The world needs to build the battery factories before we can replace all cars with electric.
Also figure in car factory capacity. There is some excess capacity in the car industry - if all factories worked to capacity they could probably built 50% more in a year than they do normally, but that isn't enough to replace every car in use in a year. If we have cars on the road today built 20 years ago, then in 20 years time we will have cars on the road built today. Building the extra factories just to produce more cars for a very short time to enable the change would be a huge waste of money, concrete, and other resources.
To be perfectly honest, some of the government timescales are not achievable unless the process denies poorer people the ability to own cars, which would be too unpopular for most governments to consider implementing.