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Re: Re: Unexplained Behavior in Date::Calc

by Madams (Monk)
on Dec 02, 2003 at 01:57 UTC ( #311487=note: print w/ replies, xml ) Need Help??


in reply to Re: Unexplained Behavior in Date::Calc
in thread Unexplained Behavior in Date::Calc

I've just got to give you a ++ for the pointer to Perl DateTime Suite,I just browsed it, now I'm gonna try it.

Also when stating that the module is working as advertised, Delta_DHMS() does, right in its name, tell you that it DOESN'T know or care about years; else it would have been named Delta_YDHMS(). That said all the functions should use absolute dates. (It's a feature;) .)


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Comment on Re: Re: Unexplained Behavior in Date::Calc
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Re^3: Unexplained Behavior in Date::Calc
by mojotoad (Monsignor) on Dec 02, 2003 at 06:33 UTC
    Both Delta_DHMS() and Delta_YMDHMS() correspond to their companion functions Add_Delta_DHMS() and Add_Delta_YDHMS(). In both cases, the results from Delta_* can be passed to the Add_Delta_* function, along with the first date, in order to get the second date. Accordingly, the complimentary operation is supported if you reverse the sign of each value of the delta and pass to Add_Delta_* along with the second date.

    From the POD:

    In any case, adding the result of this function to the first date/time + value ($year1,$month1,$day1, $hour1,$min1,$sec1) always gives the se +cond date/time value ($year2,$month2,$day2, $hour2,$min2,$sec2) again +, and adding the negative result (all elements of the result vector n +egated) to the second date/time value gives the first date/time value +.

    As you can see, the specification for what Delta_YMDHMS() and Delta_DHMS() return is defined in terms of complimentary operations, not any particular representation of values. So in your example (hr/day labels fixed):

    Today_and_Now value: 2003-12-1-14-52-15
         End time value: 2004-4-1-0-0-0
    
    Function     |Yrs  |Mons |Days |Hrs  |Mins |Secs
    -------------------------------------------------
    Delta_DHMS   |  N/A|  N/A|  121|    9|    7|   45
    Delta_YMDHMS |    1|   -8|    0|  -14|  -52|  -15
    
    Add_Delta_YMDHMS() (no modulo, no carry):
    
      original: 2003 |  12 |   1 |  14 |  52 |  15
    +    delta:    1 |  -8 |   0 | -14 | -52 | -15
    -------------------------------------------------
           end: 2004 |   4 |   1 |   0 |   0 |   0
    -    delta:    1 |  -8 |   0 | -14 | -52 | -15
    -------------------------------------------------
      original: 2003 |  12 |   1 |  14 |  52 |  15
    
    Add_Delta_DHMS() (modulo, columnar carry, calendrical):
    
          math:|   calendrical   |mod24|mod60|mod60
    -------------------------------------------------
      original:       2003-12-01 |  14 |  52 |  15
    +    delta:              121 |   9 |   7 |  45
    -------------------------------------------------
           end:       2004-04-01 |   0 |   0 |   0
    -    delta:              121 |   9 |   7 |  45
    -------------------------------------------------
      original:       2003-12-01 |  14 |  52 |  15
    
    
    As you can see, there's no actual calendrical or modulo/carrying calculations going on with Delta_YMDHMS(). Delta_DHMS(), on the other hand, does do it that way.

    Nowhere in the documentation does it state that the Delta_* functions perform their arithmetic in the same fashion, or represent their results the same way. They are merely guaranteed to provide symmetry for the Add_Delta_* functions.

    Matt

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