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Julian and calendar date subroutines

PostPosted: Mon Jul 20, 2015 6:44 am
by cdeaglejr
Here are two EzSBC Control BASIC subroutines that can be used to calculate Julian and calendar dates. The source code listing includes a short program that demonstrates how to interact with these two subroutines.

Note that the time between two calendar dates is the difference between their corresponding Julian dates.

Additional information about dates can be found at my Orbital and Celestial Mechanics Website located at

http://celestialandorbitalmechanicswebs ... asite.com/

Code: Select all
' dates.bas

' this program demonstrates how to interact
' with the julian and gdate subroutines
'
' July 20, 2015

'''''''''''''''

pi2 = 2.0 * PI

' user-defined define calender date

month = 6
day = 16.2468642
year = 2015

' print calendar date

PRINT " "
PRINT "user-defined calendar date"
PRINT " "
PRINT "month = ", month
PRINT "day   = ", day
PRINT "year  = ", year

' call julian date subroutine

GOSUB JULIAN:

' print julian date

PRINT " "
PRINT "jdate = ", jdate

' call calendar date subroutine

GOSUB GDATE:

PRINT " "
PRINT "calendar date computed from julian date"
PRINT " "
PRINT "month = ", month
PRINT "day   = ", day
PRINT "year  = ", year

END

''''''''''''''''
''''''''''''''''

JULIAN:

' Julian date subroutine

' Input

'  month = calendar month [1 - 12]
'  day   = calendar day [1 - 31]
'  year  = calendar year [yyyy]

' Output

'  jdate = Julian date

' special notes

'  (1) calendar year must include all digits

'  (2) will report October 5, 1582 to October 14, 1582
'      as invalid calendar dates and set jdate = 0

''''''''''''''''''''''''''''''''''''''''''''''''''

y = year

m = month

b = 0.0

c = 0.0

IF (m <= 2.0) THEN

   y = y - 1.0

   m = m + 12.0

ENDIF

IF (y < 0.0) THEN

   c = -0.75

ENDIF

' check for valid calendar date

IF (year < 1582.0) THEN

   ' null

ELSE

IF (year > 1582.0) THEN

   a = INT(y / 100.0)

   b = 2.0 - a + INT(a / 4.0)

ENDIF

ELSE

   IF (month < 10.0) THEN

   ' null

ENDIF

ELSE

IF (month > 10.0) THEN

   a = INT(y / 100.0)

   b = 2.0 - a + INT(a / 4.0)

ENDIF

ELSE

IF (day <= 4.0) THEN

   ' null

ENDIF

ELSE

IF (day > 14.0) THEN

   a = INT(y / 100.0)

   b = 2.0 - a + INT(a / 4.0)

ENDIF

ELSE

   ' this is an invalid calendar date

   PRINT "this date does not exist"

   jdate = 0.0

   RETURN

ENDIF

jd = INT(365.25 * y + c) + INT(30.6001 * (m + 1.0))

jdate = jd + day + b + 1720994.5

RETURN

'''''''''''''''
'''''''''''''''

GDATE:

' convert Julian date to Gregorian (calendar) date subroutine

' input

'  jdate = julian day

' output

'  month = calendar month [1 - 12]
'  day   = calendar day [1 - 31]
'  year  = calendar year [yyyy]

'  note: day may include fractional part

''''''''''''''''''''''''''''''''''''''''

jd = jdate

z = INT(jd + 0.5)

fday = jd + 0.5 - z

IF (fday < 0.0) THEN

   fday = fday + 1.0

   z = z - 1.0

ENDIF

IF (z < 2299161.0) THEN

   a = z

ELSE

   alpha = INT((z - 1867216.25) / 36524.25)

   a = z + 1.0 + alpha - INT(alpha / 4.0)

ENDIF

b = a + 1524.0

c = INT((b - 122.1) / 365.25)

d = INT(365.25 * c)

e = INT((b - d) / 30.6001)

day = b - d - INT(30.6001 * e) + fday

IF (e < 14) THEN

   month = e - 1.0

ELSE

   month = e - 13.0

ENDIF

IF (month > 2.0) THEN

   year = c - 4716.0

ELSE

   year = c - 4715.0

ENDIF

RETURN

'''''''''''''''
'''''''''''''''

SIGN:

' sign of a variable subroutine

' input

'  x = argument

' output

'  sgn = sign of x

''''''''''''''''''

IF (abs(x) <= 1.0e-12) THEN

   sgn = 0.0

   RETURN

ENDIF

IF (x > 0.0) THEN

   sgn = 1.0

   RETURN

ENDIF

IF (x < 0.0) THEN

   sgn = -1.0

ENDIF

RETURN