It must be the autumn of 1983. When I visited a friend of mine (a radio amateur), he showed me his new toy, the Sinclair ZX-81. one of the cheapest home computers at the time. This was not the first time a saw a computer and I had definitely entered some BASIC commands before. A year earlier I had visited an uncle of mine who was the proud owner of an Acorn Atom. It showed text on a TV-screen. As I recall, I entered a simple BASIC program to solve quadratic equations and I played with mathematical functions that could be evaluated in BASIC. And there was a version of Space Invaders that you could load from cassette tape. This was all very fascinating and I knew that I was going to have a computer sooner or later.
But back to the ZX-81. This too could show text and simple graphics on a TV screen and it too could load games from cassettes. It too had BASIC in ROM. This could be yours for less than 200 guilders including the 16 kB RAM extension that you were almost required to have. This machine could run a simple flight simulator program and I was truly amazed that this was possible on such a simple computer.
But programming was the thing I was most interested in, more than playing games. Calculating the week day for any given date was a problem that I should be able to tackle and so I did. I think I wrote the program down on paper after I ran it successfully on the ZX81, so I could enter it later when I did get a computer. The program may have looked a bit like the listing below. The original version certainly did not have REM statements, that would be a waste of memory (and would be too tedious to type on the ZX-81) and the original was in Dutch. But the algorithm was definitely very similar to what is shown, including the long clumsy lists of IF-statements. I did not know arrays at the time. I certainly did not use a fancy formula, such as Zeller's congruence. Download the program here to try in with your own BASIC interpreter.
10 REM COMPUTE DAY OF THE WEEK 20 PRINT "ENTER DAY (1-31)" 30 INPUT D 40 IF D>=1 AND D<=31 THEN GOTO 70 50 PRINT "DAY OUT OF RANGE" 60 GOTO 20 70 PRINT "ENTER MONTH (1-12)" 80 INPUT M 90 IF M>=1 AND M<=12 THEN GOTO 120 100 PRINT "MONTH OUT OF RANGE" 110 GOTO 70 120 PRINT "ENTER YEAR (1901-2099)" 130 INPUT Y 140 IF Y>=1901 AND Y<=2099 THEN GOTO 170 150 PRINT "YEAR OUT OF RANGE" 160 GOTO 120 170 REM SET L=1 FOR LEAP YEAR, OTHERWISE LET L=0 180 LET L=0 190 IF 4*INT(Y/4)=Y THEN LET L=1 200 REM ADD NUMBER OF YEARS TO DAY, EACH YEAR STARTS A DAY LATER 210 LET D=D+Y 220 REM ADD NUMBER OF LEAP YEARS BEFORE THIS YEAR 230 LET D=D+INT((Y-1)/4) 240 REM ADD DAYS OF PRECEDING MONTHS 250 IF M>1 THEN LET D=D+31 260 IF M>2 THEN LET D=D+28+L 270 IF M>3 THEN LET D=D+31 280 IF M>4 THEN LET D=D+30 290 IF M>5 THEN LET D=D+31 300 IF M>6 THEN LET D=D+30 310 IF M>7 THEN LET D=D+31 320 IF M>8 THEN LET D=D+31 330 IF M>9 THEN LET D=D+30 340 IF M>10 THEN LET D=D+31 350 IF M>11 THEN LET D=D+30 360 REM THE FOLLOWING ADDITION WAS FOUND BY TRIAL AND ERROR 370 LET D=D+5 380 REM NOW DO MODULO 7 390 LET D=D-7*INT(D/7) 400 IF D=0 THEN LET D$="SUNDAY" 410 IF D=1 THEN LET D$="MONDAY" 420 IF D=2 THEN LET D$="TUESDAY" 430 IF D=3 THEN LET D$="WEDNESDAY" 440 IF D=4 THEN LET D$="THURSDAY" 450 IF D=5 THEN LET D$="FRIDAY" 460 IF D=6 THEN LET D$="SATURDAY" 470 PRINT D$This program is written in such a way it could run on the ZX-81. So it has only one statement per line (but a statement is allowed after IF..THEN) and every assignment statement starts with LET. Sinclair BASIC requires this, other versions do not. And ZX-81 does not have an END statement (it does have STOP, but this is not required).
The algorithm is explained as follows:
Early in 1984 our family bought a used Texas Instruments TI-99/4A computer. This also had BASIC in ROM and it had 16kB of RAM, which I knew was enough. As opposed to the ZX-81 it had a real keyboard, colour graphics and sound. It came with three game cartridges: Munch Man, TI Invaders and Parsec and we spent many hours with these games. But for me programming was the main subject and I would soon be quite disappointed.
The TI computer had its own BASIC dialect. On the one hand it had many more features than ZX-81 BASIC, but on the other hand, it lacked some features that ZX-81 BASIC did have. And I thought: if the ZX-81 has it, every other computer must have it too. Like the ZX-81, the TI supported just one statement per line, but the IF statement could not have a statement after THEN, but just a line number (and you could specify another line number with ELSE). By the way, this is what ANSI Minimal BASIC specified and many early BASIC version on mainframes and minicomputers had it that way. As my original program contained many statements after IF..THEN, this was the first stumbling block. I could rewrite the program by using additional lines between the IF statements and then skip each of these lines in the IF statement, like this:
250 IF M<=1 THEN 260 255 D=D+31 260 IF M<=2 THEN 270 ...
But I did not feel like almost doubling the number of lines in the program. But after a little bit of learning, I came up with a solution that used arrays. The program may have looked like this one
100 REM COMPUTE DAY OF THE WEEK 110 DIM H(12) 120 H(1)=0 130 FOR I=2 TO 12 140 READ D 150 H(I)=H(I-1)+D 160 NEXT I 170 DIM D$(7) 180 FOR I=1 TO 7 190 READ D$(I) 200 NEXT I 210 INPUT "ENTER DAY (1-31)";D 220 IF (D>=1) * (D<=31) THEN 250 230 PRINT "DAY OUT OF RANGE" 240 GOTO 210 250 INPUT "ENTER MONTH (1-12)";M 260 IF (M>=1) * (M<=12) THEN 290 270 PRINT "MONTH OUT OF RANGE" 280 GOTO 250 290 INPUT "ENTER YEAR (1901-2099)";Y 300 IF (Y>=1901) * (Y<=2099) THEN 330 310 PRINT "YEAR OUT OF RANGE" 320 GOTO 290 330 REM IF CURRENT YEAR IS LEAP YEAR AND WE ARE IN MARCH OR LATER, ADD A DAY 340 IF (4*INT(Y/4)<>Y) + (M<3) THEN 360 350 D=D+1 360 REM ADD NUMBER OF YEARS TO DAY, EACH YEAR STARTS A DAY LATER 370 D=D+Y 380 REM ADD NUMBER OF LEAP YEARS BEFORE THIS YEAR 390 D=D+INT((Y-1)/4) 400 REM ADD NUMBER OF DAYS IN PRECEDING MONTHS. 410 D=D+H(M) 420 REM THE FOLLOWING ADDITION WAS FOUND BY TRIAL AND ERROR 430 D=D+5 440 REM NOW DO MODULO 7 450 D=D-7*INT(D/7) 460 PRINT D$(D+1) 470 END 480 REM DAYS IN MONTHS JAN-NOV 490 DATA 31,28,31,30,31,30,31,31,30,31,30 500 REM NAMES OF DAYS 510 DATA "SUNDAY","MONDAY","TUESDAY","WEDNESDAY","THURSDAY","FRIDAY","SATURDAY"
This program, may run unmodified on the TI-99/4A, except that it required a colon after the input prompt instead of a semicolon. It only uses the form of the IF-statement with a line number after THEN and I replaced AND and OR (that TI-Basic did not have) with * and +, as was commonly done on this machine.
The algorithm itself is the same as in the first program, but it uses two arrays. Both are read from DATA lines in two loops at the start of the program.
No, the disappointment about programming on the TI computer was not caused by the need to do a bit of extra learning to port my program to it. It was caused by the way the machine could not be programmed in machine code and many of the hardware features simply could not be used without buying additional expensive cartridges.
Now most programs use a formula known as Zeller's congruence to compute the day of the week, like this program does.
10 REM COMPUIE DAY OF THE WEEK 20 DIM D$(7) 30 FOR I=1 TO 7:READ D$(I):NEXT I 40 INPUT "ENTER DAY (1-31)";D 50 IF D<1 OR D>31 THEN PRINT "DAY OUT OF RANGE":GOTO 40 60 INPUT "ENTER MONTH (1-12)";M 70 IF M<1 OR M>12 THEN PRINT "MONTH OUT OF RANGE":GOTO 60 80 INPUT "ENTER YEAR (1582-2999)";Y 90 IF Y<1582 OR Y>2999 THEN PRINT "YEAR OUT OF RANGE":GOTO 80 100 IF M<3 THEN M=M+12:Y=Y-1 110 D=D+5+INT(13*(M+1)/5)+Y+INT(Y/4)-INT(Y/100)+INT(Y/400) 120 D=D-7*INT(D/7)+1: REM D MOD 7 + 1 130 PRINT D$(D) 140 INPUT "ANOTHER DAY (Y/N)";Y$ 150 IF Y$="Y" THEN GOTO 40 160 END 170 DATA "MONDAY","TUESDAY","WEDNESDAY","THURSDAY","FRIDAY","SATURDAY","SUNDAY"
This program implements the full Gregorian date rule, as opposed to the previous two programs. The computation of the weekday is done in just three lines: 100-120, so let's explain it
Each month has either 30 days of 31 days. A 30-day month contributes 2 to the day count (4 weeks + 2 days), a 31-day month contributes 3 to the day count. I know, February has 28 (or 29 days), but as it is the last month of the year we compute with, it will never need to be counted in the preceding months. In our example we effectively multiply the month number by 2.6 and when rounded down to integers, this will increment in steps of 2 or 3, exactly in the pattern we need for our purpose. That was Zeller's stroke of genius. Let's honour him for it! In order to see it, you have to make a table. The first month number we use is 3 (for March) and the last number we use is 14 (for February), For March, we compute INT(13*(3+1)/5) = INT(52/5) = INT(10.4) = 10. This is our starting point for the year. For April we compute INT(13(4+1))/5) = INT(65/5) = 13. The difference between 13 and 10 is 3, so the preceding month March has added 3 days to our starting point if we have a date in April (and March is indeed a 31-day month). The "Incr" column shows how many days the value has incremented with respect to the previous month and the "Days PM" column shows the number of days in the previous month.
Christian Zeller invented this method already in the nineteenth century, long before the age of computers. In his original formula he split he year Y into a century number J and a year within the century K. These numbers are smaller and easier to work with by humans. The Wikipedia article also lists a formula in which the year Y is used directly (and that's what I use). This has the additional advantage that the number cannot come out negative, as Zeller's original formula can and then it may be tricky to get the correct modulus. Also note that the multiplication and division is often mentioned as 26/10 instead of 13/5. For humans who use decimal arithmetic, a division by 10 is easier, for a binary computer it makes no difference. Many published computer implementations use the original formula and 26/10.
All three programs listed in this article run unmodified under Michael Haardt's bas and under Matrix Brandy. Any arrays are indexed starting at 1. Most BASIC interpreters will index arrays starting at 0, but they will run the programs anyway, the element at index 0 is just not used. The third program has multiple statements per line, so it will not run unmodified under BASIC versions with one statement per line, but it is easy to port. The programs are even portable to integer BASIC versions, you just have to omit the INT function, as in Integer BASIC the division operation rounds down anyway. Values are well within the range supported by 16-bit integers.
Zeller's congruence is a quick way to get the day of the week for any given date, but it is limited to just that. For many applications it is more useful to convert a given date to an integer that increases with each calendar day. For this purpose, there are formulas to convert a date to a Julian day and there is also an algorithm to convert a Julian day back to a calendar date.
In your calendar program you may need to do additional computations, for example checking that the entered date is valid (day number does not exceed the number of days in the month) and for that you need to check that the current year is a leap year. For that you probably will need a lookup table (but you could apply a Julian date formula to check that the first of the next month is higher than the given day). For printing a calendar you will also need to know the number of days in any month.
Julian day is the number of days passed since Jan 1, 4713BC 12:00 UTC. For the current date, this number is around 2.45 million. The number is a fractional number that can represent each time instant. For calendar applications you can treat it as an integer, but this will not fit into the 16-bit integer variables that many BASIC versions have. There is a formula to convert a given date (day, month, year) into a Julian date and there is an algorithm to convert this number back to a date. Take the Julian date modulo 7 and you will get the week day, Using these formulas you can compute the number of days between 2 dates or you can compute what date will be a given number of days from now.
I think the absolute minimum range for integers to perform calendar calculations using the Julian day is 24 bits (16 million) Numbers close to 10 million will occur in the given formulas with the current year. Single-precision floats (IEEE754) may just be sufficient, provided that arithmetic for integers below 16 million is implemented exactly. Some older BASIC versions that only support single precision numbers may not cut it.
The Julian day may come in handy when you want to convert dates between different calendars, for example the Gregorian calendar, the Jewish calendar and the Islamic calendar.