A Delphi TDateTime variable is actually a double precision number, with the whole number part representing the elapsed days since (I think) 31 December 1899, and the fractional part representing the time. The time is a fraction expressed as N/24 where N is in hours.
Example: If the time is 23:30, then N=23.5; 23.5/24.0 = 0.9791666666666667 So 1.9791666666666667 = 23:30:00 on 1 Jan 1900. So if you want to subtract 8 hours, you actually have to subtract "eight twentyfourths" or UTC := PacificTime - (8.0 / 24.0); Subtracting 8 from Pacific time would result in the same time, but 8 *days* before! You can use the SysUtils.EncodeTime function to do the fractions for you. In the context of UTC conversion, it might be easier to do the fractions yourself, since you usually don't need minutes, and certainly don't need seconds and milliseconds. Doing it yourself also allows you to use positive and negative hour offsets. EncodeTime only works on positive hour/min/sec/msec values. Positive and negative offsets are used in time conversion. HTH, Kevin G. McCoy --- In [email protected], "Glenn B. Lawler" <gblaw...@...> wrote: > > Emmanuel, > > > Does anyone know of a free component that converts the local time to UTC > and back to another time zone local time? > > I am not sure what your application is for these converted times, or what > time structure you are using. If you are talking about TDateTime, changing > from one timezone to another is simply a matter of subtracting your UTC > offset. For example, US Pacific time is -8, so you subtract -8 from the > current time to calculate UTC time. If it is 1 AM minus -8 is 9 AM. To > convert from UTC time to another local time, just add the offset. Since > TDateTime is a numeric (double) type, you can just calculate the number. > > If you are talking about file times, there are Windows API calls available. > GetFileTime returns the CreationTime, LastAccessTime, and LastWriteTime for > an open file handle. These times are UTC times, if the file system is NTFS. > FileTimeToLocalFileTime converts a UTC file time to the local time of the > machine. > > If you tell me more about what you are trying to do with these times, I can > tell you specifically how I would handle the situation. > > Glenn Lawler > www.incodesystems.com >
