[LEAPSECS] ] Fractional US civil time period representation is brittle

[Gerard Ashton]

On 1/21/2012 9:13 AM, Poul-Henning Kamp wrote:

> In message<4F1AC413.1010802 at 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