Some supplements.Referring to the time, the single word “今”, "去", "上个" has different meaning.
"今" means "this one"(the one I'm experiencing). When the unit is day, like 今天, it means "today"; when the unit is year, like 今年, it means "this year". "去" means "the one has just passed". But you can't combine 今 with other time-unit freely, I recommend you regarding "去年" as a proper word. "上个" means "the last one". For example “上个八月”, the timing unit is month, and each year has only one August. So in contrast to August in this year, "the last one" refers to last year's August.
In Chinese, "今" and "去" usually appear together to express the concept of "contemporary" and "old time". In this way, they are two opposite words. If you have any knowledge of Chinese poem(like the Tang Dynasty's or the Song Dynasty's), you will find this usage was very common in old Chinese literature. (It's still common nowadays. People will use it when they want to express in a literate way).