milliseconds time conversion problem

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Michael
Michael 2011-6-23
I'm trying to print the milliseconds field with the following
DS = datestr(MM,'HH:MM:SS.FFF')
the SS.FFF field is rounded up to the nearest second
ie. 59.000
So on the command line I input
K>>MM = .9833
then K>> ZZ = MM * 60.00
the returned value for ZZ on the command line is
59.00
The interpreter shouldn't know if this is anything other than a double multiplied by a double returning a double not a time value.
How do I get around this and how can I print HH:MM:SS.FFF in my code?
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Walter Roberson
Walter Roberson 2011-6-23
JD = 97.9833
>> datestr(JD,'HH:MM:SS.FFF')
ans =
23:35:57.120
Notice this is different than the times you get. I am using 2008b.
It does not appear to be single() vs double() as in my release datestr() will not accept single()
Arithmetically, consider
>> .9833 * 24*60
ans =
1415.952
That is, 0.9833 of a day is 1415 full minutes and 0.952 partial minutes. Multiply that by 60 and you get 57.12 -- 57 seconds and 120 milliseconds, just as is calculated on my system.
Please repeat those calculations on your system and see what gets returned.
Walter Roberson
Walter Roberson 2011-6-23
Note that if JD is a Julian Day then there needs to be a 1/2 day conversion and possibly a timezone conversion in order to get the local time. There are different Julian Day standards with different corrections; see http://en.wikipedia.org/wiki/Julian_day
These corrections could account for the hour discrepency (23 vs 17), but I do not see how they could account for the difference in minutes and seconds.

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回答（2 个）

Walter Roberson
Walter Roberson 2011-6-23
Michael, which MATLAB version are you using?
Also please check that none of your values are single precision numbers. In the time range of today, eps(datenum(now)) is about 11/10 of a millisecond so if you were using single precision or a far future date, you could run out of precision.
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Michael
Michael 2011-6-23
Hi Walter,
I found this under Serial Number Dates;
Working with Serial Date Numbers
A serial date number represents a calendar date as the number of days that has passed since a fixed base date.
In MATLAB, serial date number 1 is January 1, 0000. MATLAB also uses serial time to represent fractions of days beginning at midnight; for example, 6 p.m. equals 0.75 serial days. So the string '31-Oct-2003, 6:00 PM' in MATLAB is date number 731885.75.
The fractional part is the only part of interest in the time calculation. Could it be a problem with verion 2010b?
Different input value this time.....
K>> Billy= datestr(.7283912037,'HH:MM:SS.FFF')
Billy =
17:28:53.000
Walter Roberson
Walter Roberson 2011-6-24
That one is 52.9999996799961 seconds. Is it important that those 3/10-millionth of a second cause the result to be output as 52.999 ?

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Michael
Michael 2011-6-23
Hi Walter, I'm using 2010b. I was wondering about a type mismatch issue but here's another pass at the command line.
K>> datestr(.734016203670760,'ss.FFF')
ans =
59.000
Looks like something internal to the datestr f(x).
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Walter Roberson
Walter Roberson 2011-6-24
fix() and floor() truncate.
What you are looking at came in in R14, long before the R2008b that I am using, so it would not be responsible for any differences.
Walter Roberson
Walter Roberson 2011-6-24
sprintf('%02d:%02d:%02d', floor(HH), floor(MM), floor(Secs))

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