It depends on the convention. (I see it is confusing). You need to include the dividends between t and t+1. If you price just after the ex-dividend date, then it refers to the dividends that year; otherwise, the next year.
For instance, if I want to price a company that pays dividends on December 30, and I want to price it on January 1st, then the dividends correspond to the same year I am right now. If I want to price it on December 31, then it is obviously the one for next year.
For instance, if I want to price a company that pays dividends on December 30, and I want to price it on January 1st, then the dividends correspond to the same year I am right now. If I want to price it on December 31, then it is obviously the one for next year.