It is explained in this post :
What is interesting to note :
Jerusalem was destroyed in 587 BCE and not 607 BCE so the calculations for 1914 are off by 20 years
The 3.5 years only brings us to April 2018 so their calculations are off yet again.
When Jesus supposedly arrived invisibly in 1914, the International Bible students were all but dead. If we take Jesus' parable of the master giving his slaves money for them to invest, then that would make them the slave who buried the coins he received.
According to the society's literature they seem to imply that the master decided to give the slave a second chance and five years later they were chosen, but this doesn't align with the Bible version of the parable.