First of all, the time-travel premise is fun but not very well done. Why would a spirit transport Kiew to the future as a punishment? Nevertheless, there is a compelling conflict: Kiew falls in love with Bamee in 2022, but is also morally obliged to return to 1995 or his mother will die of grief, so it is going to be a poor decision whether he decides to stay in 2022 or return to 1995. (Yes, he actually does have a choice because he can return to 1995 as long as he fulfills his promise to the spirit).

Unfortunately, neither the time-travel premise nor the conflict is fully exploited. Sometimes the story veers into illogical or frivolous developments. For instance, the about-turn in the character of Bamee's stepmother is not logical at all. Then there is the time when Bamee suffers from amnesia--it's understandable that he would become cold to Kiew because he can't remember him. However, it does not make sense that his character would be so different that he would do things without thinking about the feelings of the brother he dotes on and still remembers. The beginning scene in which Kiew dreams of kissing Bamee even before he has travelled to the future where he meets Bamee is also not explained.

Overall, Past Senger isn't a totally bad BL series, but I would find it hard to come up with a strong reason to highly recommend it. If you are bored and have watched all the better BLs, why not? But there are probably more interesting things out there.