Canon T70 Film Counter

Yesterday I shot about 17 frames with my Canon T70. I turned the camera off when I was done. Today I turned the camera back on to finish the roll. The film counter was past the 17th frame. I took a few more shots until I needed to push the rewind button in. Is it possible that the lithium memory battery needs to be replaced? I don't know how long production of the T70 went for but it came out in 1984 so that's a while ago.