I was recently watching a video about what would happen if the US president tried to cancel or postpone the election. From what I gather, the 20th amendment ensures that his term will end on January 20th unless he's reelected in some meaningful way. Is it plausible that the senate and president could work together to extend the calendar year or redefine it in such a way that extends his term?
Statutory laws can redefine words for specific purposes ("in this chapter, the word 'investment' means..."). It takes approval of both houses of Congress and approval of POTUS, or a supermajority of both houses, to add or modify any statutory law. The ends of terms for Congress and POTUS/Veep are set in the Constitution. The Constitution cannot be amended by simple act of Congress; statutorily redefining "January 20" does not change the meaning of the term, with respect to the interpretation of the Constitution. In general, a simple act of Congress cannot amend the Constitution by globally changing the interpretation of words. That would require a constitutional amendment.