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11. Time and date management
The applications within the Z88 frequently need to manipulate times and dates. The Clock, Calendar and Diary are the obvious examples; also the Filer stores creation and "last updated" dates in its filing system. Hence a fairly comprehensive set of routines are provided to handle them. Before discussing the routines themselves, it is worth explaining how dates and times may be represented.
Within the machine, dates are represented by 3-byte unsigned integers which represent the number of days since the conventional day zero, which is Monday 23rd November 4713 BC! This number of days figure is consistent from the point of view of the New Style (Gregorian) calendar, so will not tally with historical dates before 14th September 1752 (Britain) or 14th October 1582 (continental Europe) unless the dates have been retrospectively corrected (like George Washington's birth date). However, it should be entirely correct from its chosen point of view, incorporating the following rules to deal with leap years:
Every year is a normal year (365 days) exceptThe accuracy range is 23/11/4713 BC to 31/12/18253 AD.
every 4th year is a leap year (366 days) except
every 100th year is a normal year except
every 400th year is a leap year except
every 3200th year is a leap year except
every 80000th year is a normal year.
Routines provided to convert between this rather inconvenient format and either:
1) A zoned integer representing a human date conveniently, split over registers as follows:
C (bits 7 to 5) the day of the week (1=Monday, 7=Sunday)2) An ASCII string, with various options (leading blanks, American format, century [ie. 88 or 1988], month in full [ie. Dec or December] etc).
C (bits 4 to 0) the day of the month (1 to 31, obviously)
B the month (1=january, 12=december)
DE a signed year number relative to 0 AD
Times are internally represented as unsigned 3-byte integers representing the number of 10ms (1/100 of a second) intervals since the start of the day. Routines exist to convert between this format and an ASCII string, again with various options. Further routines are provided to read or set the machine time and date. The available routines are as follows, their specifications can be found in "System Calls Reference":
GN_Gdt convert an ASCII string to an internal dateExample
GN_Pdt convert an internal date to an ASCII string
GN_Die convert from internal to external format
GN_Dei convert from external to internal format
GN_Gmd fetch current machine date
GN_Pmd set current machine date
GN_Gtm convert an ASCII string to an internal time
GN_Ptm convert an internal time to an ASCII string
GN_Gmt fetch current machine time
GN_Pmt set current machine time
GN_Msc convert real time to elapsed time
GN_Sdo output date and time to standard output
include "time.def" ; time & date call definitions; simple example to display the current date
include "stdio.def" ; standard I/O call definitions
push iy
pop de ; buffer for date
call_oz(GN_Gmd) ; get machine date
; DE = DE(in) + 3
push iy
pop hl ; source date
ld a, 240 ; century output, C = interfield delimiter
ld b, 15 ; expanded day and month (no AD/BC)
ld c, '.' ; interfield delimiter
call_oz(GN_Pdt) ; put date in ASCII format
xor a
ld (de),a ; null-terminate ASCII string
push iy
pop hl
inc hl
inc hl ; adjust string buffer address
inc hl ; of converted string
call_oz(GN_Sop) ; write string to std. output
call_oz(GN_Nln) ; new line
ret
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