Is December SS benefit considered as income for that year or for the following year?

I guess it depends upon what your SS 1099 lists as your Benefits for the year.  I've always thought that income is reported in the year you received it, regardless of what year it was "earned" based on the way PTO is paid on my paycheck.  If I take PTO the last week of December and am paid on December 31, that paycheck will only have my anticipated 40 hours for that last week (because they were in the process of preparing paychecks and direct deposits while I was actually on PTO).  The PTO for the last week of the year (or sick pay for that week if I was out for that reason) is paid in arrears so it's posted on the first paycheck of the next year and is reported in the taxes for the year it's received..  I'm not a tax professional and don't recall if I was told this by someone who once did my taxes or someone where I work but I've always done it that way as my W-2 also reports it that way.  I don't absolutely know how SS benefits paid in arrears are reported but I always just pull an SSA-099 and report whatever is on that.

If you think you need to pay taxes on your Social Security income you figure that out by using the form, Social Security Benefits Worksheet. For some reason I am not allowed to add an image of the form. You need to check out the IRS web site. Internal Revenue Service | An official website of the United States government (irs.gov)

  I actually had the same question that you have and noticed that no one on here answered correctly.  Last November is when I took my social.  So I received a check the third week in December according to my birthday, and in January I received a 1099 probably around the 15th of January for 2023.  So I am believing that the 1099 I will receive this year will be for December of 2024 earnings through November of 2025.