Framing or carpenters squares last a long time and are big enough that they can get stepped on or dropped or have something put them out of square sometime over their life. Fortunately it is easy to check and adjust them. To do this simply find a flat surface that you can draw lines on. This may rule out the dining room table.
First draw a straight line twice as long as the leg of the square. With the heel of the framing square in the middle of the line, draw a line up along the edge of the framing square (perpendicular to your base line). Then flip the square over with the heel in the same place and draw a similar line.
The two lines should match. If they don't then the square is not square.
To correct it is simple, but go slowly. If the square is at an angle of less than 90 degrees (bent towards itself) then take a punch and a hammer and make some small dimples near the inside corner of the square. This will expand the metal and make the legs of the square bend further apart.
If the square is wider than it should be, make the dimples near the point of the square. This will bring the legs closer together. Keep checking while you are doing this. If you don't have a punch you can get away with placing the square on a hard surface and whacking it with the corner of the hammer face. Repeat until you get a square square.