2068 Month

This program generates and displays a monthly calendar for any user-specified month and year. It implements Zeller’s-style day-of-week calculation at line 310 to determine the starting weekday, with adjustments for January and February treated as months 13 and 14 of the prior year. Leap year detection at lines 260–270 correctly handles the century and 400-year exceptions, setting February to 28 or 29 days accordingly. The calendar is laid out in a 6-row by 7-column grid using a DIM a(42) array, with TAB-based column positioning at 4-character intervals. The program concludes with a prompt to use the TS2040 thermal printer via the COPY command.

---

Program Structure

The program divides into two logical phases. Lines 1–49 handle initialization and user input: the month-name array a$(12,9) is populated, and the user is prompted for a month number (mo) and a year (yr) with FLASH-highlighted echo of the choices. Lines 200–530 perform the calendar calculation and rendering, followed by a printer hint at line 540 and a STOP at line 550. Line 9999 holds a SAVE with an auto-run directive.

Day-of-Week Algorithm

The starting weekday is computed at lines 290–340 using a variation of Zeller’s congruence. Lines 290–300 adjust January and February to months 13 and 14 of the previous year, a standard move to keep the formula continuous across year boundaries. The core expression at line 310 is:

x = d + 2*mo + 2 + INT((3*mo+3)/5) + INT(yr/4) + yr - INT(yr/100) + INT(yr/400)

The fractional part of x/7, multiplied by 7 and rounded, gives a day index 0–6 (line 320–330). Line 350 remaps 0 to 7, placing Sunday as day 1 of the week header “SUN MON TUE WED THU FRI SAT”.

Leap Year Detection

Lines 260–280 implement the full Gregorian leap-year rule in two steps:

• Line 260: divisible by 4 but not by 100 → leap year
• Line 270: divisible by 400 → leap year (overrides the century exclusion)

February’s day count is then set to 29 at line 280 if leap=1.

Calendar Grid Layout

A 42-element numeric array a(42) (6 weeks × 7 days) is filled from position start onward with day numbers 1–end; all other slots remain 0 from DIM initialization. The rendering loop (lines 440–530) iterates over 6 rows (x=0 TO 5) and 7 columns (c), using TAB(4*cc) for 4-character column spacing. Zero-valued cells and cells beyond end print a space to blank that position, while valid days print the day number.

Month-Name Array

a$(12,9) stores all month names padded to exactly 9 characters, matching “SEPTEMBER” (the longest). The variable mon is saved at line 203 before mo is modified by the Zeller adjustment, so the correct name can be retrieved for display at line 410.

Notable Techniques and Idioms

• FLASH 1 / FLASH 0 used to draw attention to user-echoed input values before proceeding.
• Multiple PAUSE statements (120, 180 frames) provide paced reading time without requiring a keypress.
• The separate mon and year variables preserve original user input while mo and yr are mutated for the algorithm.
• PRINT TAB(4*cc);a(c); with a trailing semicolon keeps the cursor on the same line across the inner column loop, with an explicit PRINT (line 525) to terminate each row.

Bugs and Anomalies

• Line 6 contains extraneous byte sequences (\\{20}\\{1}\\{20}\\{1}\\{20}\\{0}) embedded mid-statement before the array reference, likely a data corruption artifact in the saved file; the assignment itself functions correctly.
• The program spells “CALENDER” (line 47) where “CALENDAR” is standard American English — a typographic error in the original entry.
• No input validation is performed on mo (expected 1–12) or yr; entering out-of-range values will produce incorrect or error results.
• Lines 17, 47, and 540 use mid-string line-breaking with spaces to wrap text across the 32-column display, a common but fragile manual formatting technique.