CASIOTRON CALENDAR 200 (model number: 47CS-23) was released by Casio in the late 1970s.
"CASIOTRON" is the nickname given to Casio's earliest digital watches.
The serial number on the back cover starts with "5", so it looks like it was made in 1975, but it appears to be a model from before the rules for serial numbering were established, so it is actually a model from around 1978.
The seconds display is a type in which the gauge above the time display flashes and advances one frame every 10 seconds.

Casio previously introduced CASIO A201, it wasn't until the 1980s that a two-layer LCD structure like this was realized, so until then it was thought to be impossible to switch between clock and calendar displays on a single LCD screen.
However, with this model, Casio has managed to create a clock and calendar display on a single LCD screen.
The method is...

This is a clever method of displaying a digital display between each number (date) on the calendar display.
That's why each date on the calendar is strangely small.
The big numbers on the calendar represent the "month", and the "horizontal line" above the calendar represents the "Sunday Indicator", that is, the position of Sunday.
The display position from the 1st to the 31st is fixed, but it is displayed until the 30th in June (on the left) and until the 31st in December (on the right), which is strangely detailed.
Leap years can also be set.(When setting the time, enter the remainder after dividing the Western calendar by 4)
By the way, the "200" in the model name means "200 years of calendar built-in," which means that the years 1900 and 2100 are divisible by 4, but are not leap years, so no calendar correction is necessary between 1901 and 2099.
At the time of release of this model, more than 1/3 of "200" has already been completed.

This calendar function may seem convenient at first glance, but it has a fatal flaw.
I can see the month and Sunday position on the calendar screen, but I can't see the day, so I have no idea what day of the week it is.
When I switch to the date display to check today's date (on the left side of the photo above)...huh?"SA" is displayed in the upper right corner.
There's no point in displaying the calendar...