First (?) shown in PLINC’s Alphabet Yearbook 1969, alongside Cenotaph, a skewed dimensional derivative. One Line (1971) shows 8 variants of Calendar; 2 flat ones (Black and Outline) and 6 dimensional ones (B–H). Credited to Ed Benguiat in Photo-Lettering’s Alphabet Thesaurus Vol. 3 catalog (1971).
There’s an anonymous digitization of the outlined style.