(1) I am working in this office now (2) I have worked in this office since 2012 In (1), this tells us that you started working in this office sometime before now and that you will continue for some time in the future, but the emphasis is on the present - you are working now. In (2), this tells us that you started working in this office in 2012 and that you still work there today. It is possible that you will work there in the future also, but it is not definite - more information would be needed to know about the future. There are other usages of both tenses but this illustrates one difference.