Q: Is there a way to read Greenwich Mean Time offsets from
the Map control panel?
A: There's actually a system-level call to find out where
you are. It's a Script Manager call named
ReadLocation (used
by the Map control panel), which returns a structure giving
you all the information you need. Here's a description of
the call, copied from MPW 411:
pascal void ReadLocation(MachineLocation *loc)
= {0x205F,0x203C,0x000C,0x00E4,0xA051};
File {CIncludes}script.h
In C:
pascal void ReadLocation(MachineLocation *loc);
pascal void WriteLocation(const MachineLocation *loc);
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These routines access the stored geographic location and
time zone information for the Macintosh from parameter RAM.
For example, the time zone information can be used to derive
the absolute time (GMT) that a document or mail message was
created. With this information, when the document is
received across time zones, the creation date and time are
correct. Otherwise, documents can appear to be created after
they're read. (For example, someone could create a message
in Tokyo on Tuesday and send it to Cupertino, where it's
received and read on Monday.) Geographic information can
also be used by applications that require it.
The gmtDelta field represents the current
offset (in seconds) from Universal (Greenwich Mean) Time to the
current time on the Macintosh. So, if the current Macintosh
time is 1:00 AM and current GMT is 2:00 AM, then
gmtDelta should be -3600, since the current time
is one hour (3600 seconds) less than GMT. The
gmtDelta field is a signed three-byte value.
The high bit (bit 7) of the dlsDelta field
represents the current state of daylight savings time. If
daylight savings time is in effect, then bit 7 of
dlsDelta is set; if daylight savings time is not
in effect, then bit 7 is clear. The other 7 bits (bits 0
through 6) of dlsDelta are reserved
for future use by Apple. (This was defined for the release
of AOCE; the daylight savings time checkbox in the System 7
Pro Date & Time control panel uses these semantics as do
several other utilities.)
The value in gmtDelta always represents current
offset from GMT, regardless of the setting in
dlsDelta; GMT can always be calculated by
subtracting gmtDelta from the
current time. That means that during daylight savings time,
the value in gmtDelta will be different from the
value during standard time.
If the MachineLocation has never been set, it
should be <0,0,0>. The top byte of the
gmtDelta should be masked off and preserved when
writing: it's reserved for future extension. The
gmtDelta is in seconds east of GMT; for
example, San Francisco is at minus 28,800 seconds (8 hours *
3600 seconds per hour). The latitude and longitude are in
fractions of a great circle, giving them accuracy to within
less than a foot, which should be sufficient for most
purposes. For example, Fract values of
1.0 = 90deg., -1.0 = -90deg., and -2.0 = -180deg.
In C:
struct MachineLocation {
Fract latitude;
Fract longitude;
union {
char dlsDelta; /*signed byte; daylight savings delta*/
long gmtDelta; /*must mask - see documentation*/
} gmtFlags;
};
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The gmtDelta is really a three-byte value, so
you must take care to get and set it properly, as in the
following C code examples:
long GetGmtDelta(MachineLocation myLocation)
{
long internalGMTDelta;
internalGMTDelta = myLocation.gmtDelta & 0x00ffffff;
if ( (internalGMTDelta >> 23) & 1 ) // need to sign extend
internalGmtDelta = internalGmtDelta | 0xff000000;
return(internalGmtDelta);
}
void SetGmtDelta(MachineLocation *myLocation, long myGmtDelta)
{
char tempSignedByte;
tempSignedByte = myLocation->dlsDelta;
myLocation->gmtDelta = myGmtDelta;
myLocation->dlsDelta = tempSignedByte;
}
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Updated: 17-May-1999
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