Another Calendar thread :O

[xArt]

Hi Guys,

It’s been a while since posting here, but I came by an old closed thread while Googling:
Writing a program to make a calendar

I’m trying to write a calendar program in BASIC that receives year, month,day, and also the progressive input of a tick per day from a clock to increment each of the 3 date variables as appropriate.
I’m not concerned at all about formatting of the calendar for printing.

It occurs to me that no matter, I still have to iterate days of year, etc.
taking more time to run this function as time goes by...

int day_of_month(int month, int year)
{
    /* 1-1-1 was a monday */
    for(number of years)
    {
        day =  (day + days_in_year(year) )% 7;
    }
    for(number of months)
    {
        day = (day + days_in_month[month])%7;
        /* days_in_month array is updated according to the current 'year' variable in days_in_year()*/
    }
    return day;
}

It seems no matter how it’s done, if it were part of a clock that also updated a HH:MM:SS display, it would always glitch the display at the stroke of midnight each day.
I suppose on modern platforms this could be recalculated and stored in RAM,
but could anyone shed some light on how it was done on old computers if the platform had no real time clock?
Or did they just not keep the time/date if they lacked a real time clock chip?

Cheers, Art.

[Jimmy]

<snip>

I’m trying to write a calendar program in BASIC that receives year, month,day, and also the progressive input of a tick per day from a clock to increment each of the 3 date variables as appropriate.

<snip>

This subforum is about C programming and I see no question relating to C in your post, so I think you are in the wrong forum buddy.

[laserlight]

This subforum is about C programming and I see no question relating to C in your post, so I think you are in the wrong forum buddy.

Indeed.

*moved to Tech Board*

[xArt]

Ok C then (because it makes no difference).

I solved my own problem best I think I can.
You only have to run all the routines when the date is first set.

From there you only have to check leap year and populate number of days array when the year changes.
When the month changes you have to move the days in month index to look at days in the next month.
When the day changes you only have to check it didn’t exceed the days in the month,
and increment/reset the day of the week.

[grumpy]

If you have a criterion for deciding if a year is a leap year, you can also implement some function that computes the number of times 29 Feb occurs between two dates (without necessarily looping).

[xArt]

It didn’t turn out near as slow as I thought.
I ended up just running the entire algo for every time the day ticks over.

The only problem encountered is I’m counting the total days into a 16 bit value,
so the range is about 175 years or so before the word overflows for finding the day of the week.
I limited the year range from 1990 (which also starts on Monday) and 2150.

If I were that keen I could get a small table of years that also start on a Monday to
begin iteration from, but it should last longer than I live as it is

If you have a criterion for deciding if a year is a leap year, you can also implement some function that computes the number of times 29 Feb occurs between two dates (without necessarily looping).

[xArt]

So something like this:

' increment date if day changed over
'
if DayChanged = 1 then
calday = calday + 1
if calday > caldim[calmonth-1] then
calday = 1
calmonth = calmonth + 1
endif
if calmonth > 12 then
calmonth = 1
calyear = calyear + 1
endif
'
DayChanged = 0
gosub setcal
endif ' DayChanged
'
‘
goto main
‘

This is the calendar that is called as a subroutine called “setcal”:

'
' ******************* set calendar date and find day of week *******************
'
setcal:						' find leap year status
caldim[0] = 31				' set default days in month table
caldim[1] = 28				' days in February is changed to 29 by the program
caldim[2] = 31				'
caldim[3] = 30				'
caldim[4] = 31				'
caldim[5] = 30				'
caldim[6] = 31				'
caldim[7] = 31				'
caldim[8] = 30				'
caldim[9] = 31				'
caldim[10] = 30				'
caldim[11] = 31				'
'
calleap = 0					' clear leap year status
if calyear // 4 = 0 then	'
calleap = 1					' set leap year status
if calyear // 100 = 0 then	'
calleap = 0					' clear leap year status
if calyear // 400 = 0 then	'
calleap = 1					' set leap year status
endif						'
endif						'
endif						'
'
if calleap = 1 then			'
caldim[1] = 29				'
endif						'
'
calbadentry = 0				' reset calendar bad entry status
if calmonth = 0 then		' check for invalid date input
calbadentry = 1				'
calmonth = 1				'
endif						'
if calmonth > 12 then		'
calbadentry = 1				'
calmonth = 1				'
endif						'
if calday = 0 then			'
calbadentry = 1				'
calday = 1					'
endif						'
if calday > caldim[calmonth-1] then
calbadentry = 1				'
calday = 1					'
endif						'
if calyear < 1800 then		' implement lower year range restriction
calbadentry = 1				' prevent far away dates slowing program down
calyear = 1800				'
endif						'
if calyear > 2300 then		' implement higher year range restriction
calbadentry = 1				' in case of errors far into the future
calyear = 2300				'
endif						' the hardware probably won't last that long
'
' day of week
'
xwork = 2310					' find best epoch year to iterate from
cacnt = 12						'
while xwork >= calyear			'
gosub epochyears				'
cacnt = cacnt - 1				'
wend							'
'
'
cwork = 0 ' working variable for days
'
for cacnt = xwork to calyear - 1' iterate years from epoch year
calleapx = 0					' clear leap year status
if cacnt // 4 = 0 then			'
calleapx = 1					' set leap year status
if cacnt // 100 = 0 then		'
calleapx = 0					' clear leap year status
if cacnt // 400 = 0 then		'
calleapx = 1					' set leap year status
endif							'
endif							'
endif							'
'
if calleapx = 0 then			' add days over the years from epoch
cwork = (cwork + 365)//7		'
else							'
cwork = (cwork + 366)//7		'
endif							'
'
next cacnt						'
'
if calleap = 1 then				' fix day of month table for current year
caldim[1] = 29					'
else							'
caldim[1] = 28					'
endif							'
'
cacnt = calmonth-1				' add days for elapsed months of current year
while cacnt > 0					'
xwork = caldim[cacnt-1]			'
cwork = (cwork + xwork)//7		'
cacnt = cacnt - 1				'
wend							'
'
cacnt = calday-1				' add days elapsed of current month
xwork = 0						'
while cacnt > 0					'
xwork = xwork + 1				'
cacnt = cacnt - 1				'
wend							'
cwork = (cwork + xwork)//7		'
'
'
caldow = cwork + 1				' shift range for display so 0=Sunday, 6=Saturday.
if caldow = 7 then caldow = 0	'
return
'
'
epochyears:
lookup2 cacnt,[1798,1849,1900,1951,1990,2035,2052,2091,2125,2153,2198,2227,2244],xwork
return
'