Dmitri O.Kondratiev wrote:
I have: ds1 = "10/11/2009 7:04:28 PM" ds2 = "10/17/2009 8:48:29 AM" t1 = readTime defaultTimeLocale "%m/%d/%Y %l:%M:%S %p" ds1 :: UTCTime t2 = readTime defaultTimeLocale "%m/%d/%Y %l:%M:%S %p" ds2 :: UTCTime dif = diffUTCTime t2 t1
I need to: 1) Find how many complete days elapsed between t1 and t2
It depends what you mean by "complete days". If you just mean how many periods of 24 hours of UTC, without caring about things like local clock changes for summer time, then it's just this: floor $ dif / (24 * 60 * 60) Otherwise, see below.
2) Find UTCTime for a moment 6 days after t1, in other words time equal to t1 + 6 * 24 hours.
addUTCTime (6 * 24 * 60 * 60) t1
Questions: 1) Is there any library function that will convert (diff = diffUTCTime t2 t1) to a data structure similar to a tuple (days, hours, mins, secs) where 'days' is a whole number of days in my 'diff'' interval, and similar for 'hours', 'mins' and 'secs' in the tuple above?
If what you mean is amounts of UTC time, then just use the diffs as floating point numbers. Divide and floor, as above. You don't need any special functions. As you can see, things are easy as long as you stay in UTC. Often that is a little sloppy, but if it is good enough for your program, use it.
2) What is the 'right way' of adding days to UTCTime? Should I add just equivalent number of seconds or should I convert UTCTime to Data.Time.Calendar.Day type first and then use 'addDays' function?
If what you need is "days", "hours", etc. as concepts of local time, and you care about things not going wrong in the unusual case of a change of clock, then yes, you need to convert to local time and do your calculations there.
3) How to convert UTCTime to Data.Time.Calendar.Day and back to UTCTime?
If you stay in UTC, it's easy: a UTCTime is made up of a Day and a DiffTime. Suppose you need "days" in a certain time zone, not UTC. If you already know for each of your input times whether it is standard time or summer time in that time zone, then here is an outline of how: 1. Create a TimeZone for each of standard time and summer time. 2. Use utcToLocalTime with the appropriate TimeZone to convert each input UTCTime to a LocalTime. 3. Use diffDays. If all you care about is days, compare the TimeOfDay with < or > to see if you need to adjust the number of days by one. Or work with the hours, minutes, and seconds of the TimeOfDay directly. If you don't already know whether your input times are in summer time, then you need the timezone-olson and timezone-series packages. Here is an outline: 1. Get the Olson time zone file provided by your operating system for the time zone in question. On Linux and Mac OS X, it's in /usr/share/zoneinfo. On Windows it's in a binary format in the registry, Unfortunately, we don't have a parser for that format yet, so your best bet for now is to get the Olson files you need from a non-Windows computer. 2. Use a function in the timezone-olson package to read the Olson file and get a TimeZoneSeries. 3. Use utcToLocalTime' from the timezone-series package to get a LocalTime for each of your input times. Then continue as in #3 above. Actually, we started out with parsing a time string, and we have been assuming all along that it represented UTC. As always, that is the easy case. You're also OK if the time string has an explicit offset from UTC, like "-0400" for EDT in the U.S. Otherwise, you'll have to parse those strings as LocalTime, not UTCTime. If the LocalTime is what you need, great. If not, use a TimeZone or TimeZoneSeries, as above, to get back to UTCTime. Hope this helps, Yitz