I'm not sure once in how many years the census is conducted in each country to determine the population. I guess it is around once every 5 years.
To arrive at the number by which the population of the World is increasing everyday, experts don't have to count the number of people born each day and subtract the number of people that die each day from it. That is something that would be impossible to do.
A rough increase in the World population everyday can be estimated using other methods. Experts can take the latest population figure of each country and subtract from it the population figure during the previous census. The increase in population can then be divided by the number of days that have passed between the two census. This gives the increase in population everyday for each country. The sum of this for all the countries would give a rough estimate of how much the population of the World is increasing by everyday.