The Day You Were Born

This program calculates the day of the week and astrological sign for any user-supplied date of birth, using Zeller’s congruence adapted for BASIC. The zodiac sign is determined by a computed GO TO branching directly to line numbers constructed as month×100+day, with dedicated lines for each sign boundary. January and February births require a year-minus-one adjustment (line 2000), splitting year input across two branches before rejoining at line 3000. The century and year-within-century are extracted separately from a four-character string input to feed Zeller’s formula.

Program Structure

The program is divided into several logical phases:

1. Splash screen and safety check (lines 2–3): Displays the title and authorship, then PEEKs a system variable to decide whether to proceed or auto-save.
2. Input collection (lines 10–16): Gathers day, month, and year with basic range validation. January/February births are diverted to line 2000 for the Zeller adjustment.
3. Zodiac dispatch (lines 20–1299): A computed GO TO ms*100+j jumps to one of thirteen labelled lines that PRINT the sign name.
4. Day-of-week output (lines 1300–1326): Clears part of the screen and prints the day name from a cascade of IF z=n tests.
5. Zeller’s calculation (lines 3000–3060): Extracts century and year from the string, applies the formula, and branches to line 18 — which does not exist (see Bugs).
6. Save/verify (line 9999): Stores the program to tape with LINE 1 autostart.

Key BASIC Idioms

• Computed GO TO: GO TO ms*100+j at line 30 is the central dispatch mechanism. Line numbers encode the month-start boundary of each zodiac sign (e.g., line 1121 = 11th month, 21st day = Scorpio starts 21 November).
• LINE input for year: Using INPUT … LINE a$ accepts the year as a string, allowing it to be sliced with a$(1 TO 2) and a$(3 TO 4) at lines 3010–3020 to separate century from year without string-to-number conversion ambiguity.
• Cascade of IF statements for day names (lines 1320–1326) rather than an array or DATA block — typical of early hobbyist BASIC where string arrays were cumbersome.
• VAL on substrings: VAL a$(1 TO 2) is a compact way to convert a two-digit slice to a number.

Zeller’s Congruence

Lines 3030–3040 implement Zeller’s congruence for the Gregorian calendar:

• c = century (first two digits of year)
• a = year within century (last two digits)
• m = adjusted month (March = 1 … February = 12; January/February treated as months 11/12 of the previous year)
• Formula: x = INT(2.6*m - 0.199) + j + a + INT(a/4) + INT(c/4) - 2*c
• z = INT x - 7*INT(x/7) extracts the remainder mod 7 without the MOD operator (not available in standard Spectrum BASIC).

The January/February diversion at line 2000 sets m = ms + 10 (so January → 11, February → 12) and prompts for “Year of birth −1”, correctly decrementing the year for these months as Zeller’s algorithm requires.

Notable Techniques

Content

Appears On

Byte Power September 1986

Tape-based magazine.

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The Day You Were Born

You enter your date of birth and the computer will tell you on what day of the week you were...

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Source Code

2 CLS : PRINT AT 8,5;"The Day You Were Born";AT 10,2;"Written by KRISTIAN BOISVERT";AT 12,8;"©1986 BYTE POWER"
    3 IF PEEK 23681=0 THEN PRINT AT 15,0;: LIST 9999: STOP 
   10 INPUT "Day of birth:";j: IF j>31 OR j<1 THEN GO TO 10
   11 INPUT "Month of birth:";ms: IF ms<1 OR ms>12 THEN GO TO 11
   12 LET m=ms-2: IF ms<=2 THEN GO TO 2000
   15 INPUT "Year of birth:"; LINE a$: IF LEN a$<>4 THEN GO TO 15
   16 GO TO 3000
   20 CLS 
   30 PRINT AT 5,0;"Your sign is:";: GO TO ms*100+j
  119 PRINT TAB 13;"Capricorn"
  217 PRINT TAB 13;"Aquarius"
  320 PRINT TAB 13;"Pisces"
  419 PRINT TAB 13;"Aries"
  520 PRINT TAB 13;"Taurus"
  620 PRINT TAB 13;"Gemini"
  722 PRINT TAB 13;"Cancer"
  822 PRINT TAB 13;"Leo"
  922 PRINT TAB 13;"Virgo"
 1022 PRINT TAB 13;"Libra"
 1121 PRINT TAB 13;"Scorpio"
 1221 PRINT TAB 13;"Sagittarius"
 1250 PRINT TAB 13;"Capricorn"
 1300 FOR x=6 TO 17: PRINT AT x,13;"           ": PAUSE 10: NEXT x
 1310 PRINT AT 7,0;"And you were born on a ";
 1320 IF z=0 THEN PRINT "SUNDAY"
 1321 IF z=1 THEN PRINT "MONDAY"
 1322 IF z=2 THEN PRINT "TUESDAY"
 1323 IF z=3 THEN PRINT "WEDNESDAY"
 1324 IF z=4 THEN PRINT "THURSDAY"
 1325 IF z=5 THEN PRINT "FRIDAY"
 1326 IF z=6 THEN PRINT "SATURDAY"
 1330 INPUT "Another? "; LINE q$: IF q$="n" OR q$="N" THEN STOP 
 1340 RUN 
 2000 LET m=ms+10
 2010 INPUT "Year of birth -1:"; LINE a$: IF LEN a$<>4 THEN GO TO 2010
 3000 CLS 
 3010 LET c=VAL a$(1 TO 2)
 3020 LET a=VAL a$(3 TO 4)
 3030 LET x=INT (2.6*m-.199)+j+a+INT (a/4)+INT (c/4)-2*c
 3040 LET z=INT x-7*INT (x/7)
 3060 GO TO 18
 9999 SAVE "THE DAY" LINE 1: VERIFY "THE DAY"

Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.