Z88 Developers' Notes
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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) except
            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.
The accuracy range is 23/11/4713 BC to 31/12/18253 AD.

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)
            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
2) An ASCII string, with various options (leading blanks, American format, century [ie. 88 or 1988], month in full [ie. Dec or December] etc).

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 date
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
include "#stdio.def"             ; standard I/O call definitions
; simple example to display the current date
; assumes IY points to at least 30 bytes of available memory
; this memory should not lie in segment 2
            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

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Filters Time and date management Integer arithmetic