*Zellerday v0.1
*24/12/2020
*Author Brian Gaze
*Zeller's rule


*Pass in dayofmonth, month, year


function main(args)
*'reinit'
* say "temps.gs " args
* Get arguments
if (args='?')
return
else
day=subwrd(args,1);if(day='');day="";endif
month=subwrd(args,2);if(month=''); month="";endif
year = subwrd(args,3);if(year=''); year="";endif
endif



****day of the month k
k=day

*****get m which is the month number,year
***** return day
if month = MAR
m= 1
endif

if month = APR
m= 2
endif

if month = MAY
m= 3
endif

if month = JUN
m= 4
endif

if month = JUL
m= 5
endif

if month = AUG
m= 6
endif

if month = SEP
m= 7
endif

if month = OCT
m= 8
endif

if month = NOV
m= 9
endif

if month = DEC
m= 10
endif

if month = JAN
m= 11
endif

if month = FEB
m= 12
endif

*****Get last 2 digits of the year
D = substr(year,3,2)

*Subtract 1 from year if month is January or February
if m=11
D=D-1
endif

if m=12
D=D-1
endif


*****C is first 2 digits of the year
C=20


*get remainder of [(13*m-1)/5]
xz =  (13*m-1)/5

xzform=xz

if valnum(xzform) =2
xzlen = strlen(xz)
xz1dp = math_format('%.1f',xz)
*substr (string, start, length)
xzform= substr(xz1dp,1,xzlen-2)
endif


*get remainder of [D/4]
xzd= D/4
xzdform=xzd
if valnum(xzdform) =2
xzdlen= strlen(xzd)
xzd1dp = math_format('%.1f',xzd)
xzdform= substr(xzd1dp,1,xzdlen-3)
endif



*f = k + [(13*m-1)/5] + D + [D/4] + [C/4] - 2*C
f = k + xzform + D + xzdform + C/4 - 2*C

*grads command
*rc = math_mod(num1,num2);
*num1       any number
*num2       any number not equal to zero
*rc         the integer part of the remainder when num1 is divided by num2

if f>-1
daynum = math_mod(f,7)
endif

if f<0
daynum = math_mod(f,-7)
endif

if daynum<0
daynum = daynum +7
endif

if daynum=0
week = Sun
endif

if daynum=1
week = Mon
endif

if daynum=2
week = Tue
endif

if daynum=3
week = Wed
endif

if daynum=4
week = Thu
endif

if daynum=5
week = Fri
endif

if daynum=6
week = Sat
endif

return week