I actually did this once. I was clearing out unused calendar inserts and by sheer coincidence, found some that were perfect for the coming year. I just ignored the year part and used them for an entire year. I would not normally have done so, but these inserts were brilliant discontinued Mulberry design that I really liked.
I would not keep old calendars for decades waiting to be able to reuse them, but I have taped over dates and used an old Hobonichi as a notebook, which worked very well.
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