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add/subtract with date!

 [1/13] from: t_degrav::rhrk::uni-kl::de at: 14-Nov-2000 1:23


Ok, I'm not sure if this message came through the first time... :-( _____________ Hi there! I'm wondering how to calculate when using date! values... As long as I only want to add/subtract a certain amount of DAYS there seem to be no problems, but when it comes to MONTHS or YEARS it gets a little comlicated. Or am I missing something simple here??? Let's say I have:
>> date: 14-Nov-2000
== 14-Nov-2000 Now it's easy to say:
>> newdate: date + 1
== 15-Nov-2000
>> newdate: date + 100
== 22-Feb-2001
>> newdate: date - 1
== 13-Nov-2000
>> newdate: date - 400
== 11-Oct-1999 So, even roll-overs to another month or year are recognized. That's great! But how can I add/subtract exactly one month or one year? Using a certain amount of days would depend on which month I start with.
>> newdate: date + ???
== 14-Dec-2000
>> newdate: date + ???
== 14-Nov-2001
>> newdate: date - ???
== 14-Oct-2000
>> newdate: date - ???
== 14-Nov-1999 As far as I can see right now there's no way to do that directly, so I started experimenting with to-date. Now I can do (parenthesis only included for better readability):
>> newdate: to-date reduce [date/day (date/month + 1) date/year]
== 14-Dec-2000
>> newdate: to-date reduce [date/day (date/month + 10) date/year]
== 14-Sep-2001
>> newdate: to-date reduce [date/day date/month (date/year + 1)]
== 14-Nov-2001 An overflow of the month is correctly recognized and the year adjusted. But, when I try:
>> newdate: to-date reduce [date/day (date/month - 1) date/year]
== 14-Oct-2000
>> newdate: to-date reduce [date/day (date/month - 15) date/year]
== 14-Dec-23844
>> newdate: to-date reduce [date/day date/month (date/year - 1)]
== 14-Nov-1999 Wow, if I go back 15 months I jump ahead 21844 years! ;-) Ok, when I think about how 'to-date might work (or better how 'to does) I have to admit this result isn't very surprising... Final question: Is there any easy way of adding/subtracting months or years without having to check for boundaries myself??? TIA, Tom

 [2/13] from: holger:rebol at: 14-Nov-2000 9:17


On Tue, Nov 14, 2000 at 08:55:19AM -0700, Russell Yost wrote:
> Curiouser and curiouser! > A console session:
<<quoted lines omitted: 11>>
> >> > So far I haven't figured out how this happens!
Looks like correct behavior. In the second example before you change the month the year is 2002. Then you set the month to "November + 15 months", i.e. REBOL normalizes the date to "November + 15 months + year 2002" and correctly ends up with February 2004. -- Holger Kruse [holger--rebol--com]

 [3/13] from: t_degrav:rhrk:uni-kl at: 14-Nov-2000 19:00


Hi Russel, maybe I have the explanation: When you think of date as having the three components day, month and year you don't directly change the whole date with "today/month: now/month + 15" but only the month! So you get 14-26-2000, which means more or less the 14th day of the 26 month(!) of the year 2000. Rebol recognizes that this isn't a valid date, but is able to calculate a corresponding date: 14-2-2002. If you now repeat "today/month: now/month + 15" you first get 14-26-2002. This is then transformed to 14-2-2004. Maybe you confused yourself a little by mixing today and now? Try today/month: today/month + 15, then you get the result you possibly expected, the date increases exactly 15 months. This maybe confusing at first but comes in rather handy when you want to add a certain amount of months instead of days. The only thing I miss is the automatic conversion of negative month values to preceding years. Tom Russell Yost wrote:

 [4/13] from: brett:codeconscious at: 15-Nov-2000 12:03


> This maybe confusing at first but comes in rather handy when you want to > add a certain amount of months instead of days. The only thing I miss is > the automatic conversion of negative month values to preceding years. >
Yes it would be nice if that bug was fixed. Before that occurs here's a workaround. add-months: function [ "Offsets a date using a number of months (positive or negative)." date [date!] months [integer!] ] [result-date delta-years delta-months] [ delta-years: to-integer divide months 12 delta-months: remainder months 12 if lesser? delta-months 0 [ delta-years: subtract delta-years 1 delta-months: add delta-months 12 ] result-date: date result-date/year: add result-date/year probe delta-years result-date/month: add result-date/month probe delta-months return result-date ] Brett.

 [5/13] from: rryost:home at: 14-Nov-2000 8:55


Curiouser and curiouser! A console session:
>> today: now
== 14-Nov-2000/8:37:21-7:00
>> today/month: now/month + 15
== 26 ; oops! but --
>> today
== 14-Feb-2002/8:37:21-7:00 ; somehow, the error was corrected! But let's try again:
>> today/month: now/month + 15
== 26
>> today
== 14-Feb-2004/8:37:21-7:00
>>
So far I haven't figured out how this happens! Russell [rryost--home--com]

 [6/13] from: rryost:home at: 14-Nov-2000 9:05


In my preceding post, REBOL version: REBOL/Core 2.3.0.3.1 Also, note in the second "try" of today/month: today/month + 15, the year incremented by 2, but the month did not change. The result was as though I had used 24 instead of 15. Russell [rryost--home--com]

 [7/13] from: allen:rebolforces at: 14-Nov-2000 18:47


Hi Tom, REBOL makes it exceptionally easy to do. . date: 14-Nov-2000 date/day: date/day + 10 print date 24-Nov-2000 date/month: date/month + 3 print date 24-Feb-2001 date: 14-Nov-2000 date/year: date/year - 1 print date 14-Nov-1999 Only thing to watch is a bug in month subtraction. It goes flaky if you subtract prior to January . (Hopefully will be fixed soon Feedback #4014) date: 14-Nov-2000 date/month: date/month - 12 print date 14-Mar-23845 But subtraction using days does not have this problem date: 14-Nov-2000 date/day: date/day - 366 print date 14-Nov-1999 ----- >
> Final question: Is there any easy way of adding/subtracting months or
years
> without having to check for boundaries myself???
Have a look at the calendar style script on my Rebsite, you will find a comment in that script where I handle the bug. Cheers, Allen K

 [8/13] from: brett:codeconscious at: 15-Nov-2000 15:17


Oops left in a couple of unnecessary "probes" at the end of my add-months function - just delete them. Also, while we are playing around with dates I thought I'd pass on another function too. Rebol allows you to add a time to a date. Like so
>> start-time: now
== 15-Nov-2000/15:13:59+11:00
>> end-time: start-time + 2:00
== 15-Nov-2000/17:13:59+11:00 But you cannot get the time interval using a simple operator (correct me if I'm wrong)
>> end-time - start-time
== 0 But using my datetime-subtract function you can
>> datetime-subtract end-time start-time
== 2:00 Here 'tis (expanded for readability) datetime-subtract: function [ "Subract one date from another returning time between them." a [date!] b [date!] ][delta-days delta-hhmmss a-hhmmss b-hhmmss] delta-days: subtract a/date b/date a-hhmmss: either a/time [a/time][00:00:00] b-hhmmss: either b/time [b/time][00:00:00] return add multiply 24:00:00 delta-days (subtract a-hhmmss b-hhmmss) ] Brett.

 [9/13] from: jgoodnow:pacbell at: 13-Nov-2000 22:57


At 01:23 AM 11/14/2000 +0100, you wrote:
>I'm wondering how to calculate when using date! values... > >As long as I only want to add/subtract a certain amount of DAYS there >seem to be no problems, but when it comes to MONTHS or YEARS it gets a >little comlicated. Or am I missing something simple here???
Hi Tom, Try: date/month: date/month + 1 Hope this helps. - jim

 [10/13] from: arolls:bigpond:au at: 14-Nov-2000 18:57


Jim Goodnow II wrote:
> At 01:23 AM 11/14/2000 +0100, you wrote: > >I'm wondering how to calculate when using date! values...
<<quoted lines omitted: 7>>
> Hope this helps. > - jim
Jim, There is a problem with subtraction. Example:
>> date: 14-Jan-2001
== 14-Jan-2001
>> date/month: date/month - 1
== 0
>> date
== 14-Apr-23846 Anton.

 [11/13] from: t_degrav:rhrk:uni-kl at: 14-Nov-2000 12:52


Thanks, Kim, Anton, but... Anton wrote:
> Jim Goodnow II wrote:
...
> > Try: > >
<<quoted lines omitted: 12>>
> >> date > == 14-Apr-23846
That's only part of the problem... Jim's solution does look very simple, but it doesn't work at all:
>> date
== 14-Nov-2000
>> date/month: date/month + 1
== 14-Dec-2000
>> date
== 14-Nov-2000 Tom

 [12/13] from: t_degrav:rhrk:uni-kl at: 14-Nov-2000 14:08


Hi, Tom De Grave wrote:
> Jim's solution does look very simple, but it doesn't work at all: > >> date
<<quoted lines omitted: 3>>
> >> date > == 14-Nov-2000
Hm, after reading Brett's first message in the "Set-words and refinements" thread I noticed that I was still working with /core 2.2! Silly me! Well, the above DOES work with 2.3, but the problem with the boundaries when subtracting still remains! Any ideas? Thanks, Tom

 [13/13] from: brett:codeconscious at: 15-Nov-2000 0:11


On the contrary, works fine here (REBOL/View 0.10.38.3.1 31-Oct-2000)
>> date: 14-nov-2000
== 14-Nov-2000
>> date/month
== 11
>> date/month: date/month + 1
== 12
>> date
== 14-Dec-2000
>> date/month: date/month + 1
== 13
>> date
== 14-Jan-2001 But, as Jim said subtraction doesn't. BTW Jim have you submitted this to feedback? Brett.
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