Cal. 3600 was an early quartz movement from Seiko, only their second production quartz movement. It was produced only in 1970 alongside the breakthrough Cal. 35A, but quickly set aside in favor of Cal. 38.

Seiko was quietly racing to develop a quartz watch movement in the 1960s, and delivered their first Cal. 35 movement to the Neuchatel Observatory in late 1967, just a few months after the CEH delivered their "Beta" quartz movement prototypes. A product of Suwa Seikosha Cal. 35 used a 8,192 Hz of 16,384 Hz quartz crystal and an integrated circuit from Intersil in the USA. Seiko offered Cal. 35 SQ for sale in the Seiko Astron watch on Christmas Day, 1969, with 100 examples sold in the first month.

Suwa rival Daini Seikosha developed Cal. 3600 and launched it just after Cal. 35 in 1970. It was the first quartz movement to use a CMOS integrated circuit, which gave it a power consumption advantage. But Cal. 36 was still quite early to market and was retired later that year after perhaps only 1,000 examples were sold.

Variants on Cal. 36 included Cal. 3600, 3602 (36 SQC), and 3605.

Suwa's next quartz movement, Cal. 38 SQW was more production-ready and went on sale in late 1971. The Cal. 38 series marked the first true production quartz watch from Seiko.