On 1/21/2012 9:13 AM, Poul-Henning Kamp wrote:
In message<4f1ac413.1010...@comcast.net>, Gerard Ashton writes:
Consider US eastern daylight time, June 30, 2012.
[...]
This will make certain time computations quite intricate.
Representing the time as a simple floating point number may
lead to unexpected results.
If this is with reference to my "REAL" proposal, there is a fundamental
misunderstanding involved.
Only the TAI(-estimate) timestamp will be represented by a floating
point value.
All other timescales, including UTC, will be derived from this
floating point value and represented in a suitable form for that
timescale.
For instance a MJD timestamp _may_ be represented by a float, but
UTC and most forms of civil/legal/local time would be represented
with the "struct tm":
struct tm {
double tm_fraction; /* fractional part of second */
int tm_sec; /* seconds after the minute [0-60] */
int tm_min; /* minutes after the hour [0-59] */
int tm_hour; /* hours since midnight [0-23] */
int tm_mday; /* day of the month [1-31] */
int tm_mon; /* months since January [0-11] */
int tm_year; /* years since 1900 */
int tm_wday; /* days since Sunday [0-6] */
int tm_yday; /* days since January 1 [0-365] */
int tm_isdst; /* Daylight Savings Time flag */
long tm_gmtoff; /* offset from UTC in seconds */
char *tm_zone; /* timezone abbreviation */
};
The "REAL" proposal appears to include the necessary tools (when fleshed
out) but it would be
easy to misapply the tools, for example, by supposing that 1/2 UTC day
is the same duration
as 1/2 EDT day. This isn't a criticism of the idea, just an observation
on how easy it is to
go astray.
Gerard Ashton
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