Subtract a Day to Date Object Using 'myDate - 1'

[dan]

Am i breaking any rules when I loop dates like

// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}
}

I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?

TIA,
Dan

[noagbodjivictor]

On May 29, 10:55 am, [e-mail] wrote:

Am i breaking any rules when I loop dates like

// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}

}

I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?

TIA,
Dan

Is it working?

[Lee]

da*@nospam.com said:

>
Am i breaking any rules when I loop dates like

// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}
}

I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?

It certainly seems safer to use:
memDayHol.setDate(memDayHol.getDate()-1);

but the whole algorithm is nonsense.
If you want the date of the Monday before (or on) May 31, you
can calculate that without looping:

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);
--

[dan]

no*************@gmail.com wrote:

On May 29, 10:55 am, [e-mail] wrote:

>Am i breaking any rules when I loop dates like

// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}

}

I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?

TIA,
Dan

Is it working?

Yes.

[dan]

Lee wrote:

da*@nospam.com said:

>Am i breaking any rules when I loop dates like

// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}
}

I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?

It certainly seems safer to use:
memDayHol.setDate(memDayHol.getDate()-1);

but the whole algorithm is nonsense.
If you want the date of the Monday before (or on) May 31, you
can calculate that without looping:

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);

Assume Memorial Day is 5/31. Then your code will set memorial day to
5/29. I think you want

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()- (memDayHol.getDay() - 1));

Plus my code is not nonsense as it correctly calculates the Memorial day.

Dan

[Lee]

da*@nospam.com said:

>
Lee wrote:

>da*@nospam.com said:

>>Am i breaking any rules when I loop dates like

// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}
}

I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?

It certainly seems safer to use:
memDayHol.setDate(memDayHol.getDate()-1);

but the whole algorithm is nonsense.
If you want the date of the Monday before (or on) May 31, you
can calculate that without looping:

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);

Assume Memorial Day is 5/31. Then your code will set memorial day to
5/29. I think you want

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()- (memDayHol.getDay() - 1));

Plus my code is not nonsense as it correctly calculates the Memorial day.

You should learn to test. My code returns May 28.
You should also learn some mathematics:

a - b + 1 = a - ( b - 1 )

Looping to search for an answer that can be calculated is
nonsense, whether or not it produces the correct result.
--

[dan]

Lee wrote:

da*@nospam.com said:

>Lee wrote:

>>da*@nospam.com said:
Am i breaking any rules when I loop dates like

// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}
}

I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?
It certainly seems safer to use:
memDayHol.setDate(memDayHol.getDate()-1);

but the whole algorithm is nonsense.
If you want the date of the Monday before (or on) May 31, you
can calculate that without looping:

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);

Assume Memorial Day is 5/31. Then your code will set memorial day to
5/29. I think you want

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()- (memDayHol.getDay() - 1));

Plus my code is not nonsense as it correctly calculates the Memorial day.

You should learn to test. My code returns May 28.
You should also learn some mathematics:

a - b + 1 = a - ( b - 1 )

Looping to search for an answer that can be calculated is
nonsense, whether or not it produces the correct result.

Oops I thought I saw 'a - (b + 1)'. My mistake. But back to my
original question, is the '+' an undocumented way of doing day
incrementing/decrementing?

[Lee]

da*@nospam.com said:

>
Lee wrote:

>da*@nospam.com said:

>>Lee wrote:
da*@nospam.com said:
Am i breaking any rules when I loop dates like
>
// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}
}
>
I can find no docs that one can use the '+' operator directly on a date
object, and it assumes adding/subtracting a day(s). Is this deprecated
code?
It certainly seems safer to use:
memDayHol.setDate(memDayHol.getDate()-1);

but the whole algorithm is nonsense.
If you want the date of the Monday before (or on) May 31, you
can calculate that without looping:

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);
Assume Memorial Day is 5/31. Then your code will set memorial day to
5/29. I think you want

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()- (memDayHol.getDay() - 1));

Plus my code is not nonsense as it correctly calculates the Memorial day.

You should learn to test. My code returns May 28.
You should also learn some mathematics:

a - b + 1 = a - ( b - 1 )

Looping to search for an answer that can be calculated is
nonsense, whether or not it produces the correct result.

Oops I thought I saw 'a - (b + 1)'. My mistake. But back to my
original question, is the '+' an undocumented way of doing day
incrementing/decrementing?

An undocumented method is not likely to be supported by all
browsers, or even in future versions of browsers that currently
support it, so your question becomes "Does this work in some
versions of some browsers?" and that question doesn't seem to
have much value.

In both browsers that I've tried, subtracting 1 from a Date
object works as I would expect, which is not what you're
looking for. The result of subtracting 1 from
"new Date (2007, 4, 31)" is a Number, not a date: 1180594799999

A quick test is to type this as an URL in your browser:

javascript:alert((new Date())-1)

If the result is a Date object, you should see the month day
year and time of day displayed in your Locale format, instead
of the number of msec since the epoch.
--

[Dr J R Stockton]

In comp.lang.javascript message <46*********************@news.qwest.net>
, Tue, 29 May 2007 12:16:57, da*@nospam.com posted:

>Lee wrote:

>da*@nospam.com said:

>>Lee wrote:
da*@nospam.com said:
Am i breaking any rules when I loop dates like
>
// Determine Memorial Day
intFlag = 0;
memDayHol = new Date (currentYear, 4, 31);
while (intFlag == 0) {
if (memDayHol.getDay() == 1) {intFlag =1;}
else {memDayHol = memDayHol - 1;}
}
>
I can find no docs that one can use the '+' operator directly on a
>date object, and it assumes adding/subtracting a day(s). Is this
>deprecated code?
It certainly seems safer to use:
memDayHol.setDate(memDayHol.getDate()-1);

but the whole algorithm is nonsense.
If you want the date of the Monday before (or on) May 31, you
can calculate that without looping:

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);

Gives June 1 in 2009 & 2015.

>>Assume Memorial Day is 5/31. Then your code will set memorial day
to 5/29. I think you want

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()- (memDayHol.getDay() - 1));

Gives Saturdays.

>>Plus my code is not nonsense as it correctly calculates the Memorial day.

The original code, as posted, fails to run.

>Oops I thought I saw 'a - (b + 1)'. My mistake. But back to my
original question, is the '+' an undocumented way of doing day
incrementing/decrementing?

It is undocumented because assigning a number to a variable "holding" a
Date Object disconnects the Date Object and makes the variable hold a
Number.
AIUI, Memorial Day is the last Monday in May in America - elsewhere, it
may be different.

Such code should, if it is required to run in all years, be tested in
all types of year. For ordinary dates, that's 14 types - Leap or not,
starting on Mon-Sun. All types necessarily occur in any 28-year stretch
lacking a missing Leap Year.

So test in this sort of manner :-

for (Y=2000 ; Y<2030; Y++) {
memDayHol = new Date (Y, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);
document.writeln(memDayHol, "<br>") }
The easiest way to get the last X-day of a month is to think of it as
the Zeroth X-day of the following month. There is then no need to know
the length of any month.

Reading the FAQ can get you to

function NthXdayOfYM(N, X, Y, M) { var D
// X = Sun=0..Sat=6 Sun=7.. , M=1..12, N=0 for last of prev mth.
with (D = new Date(Y, M-1, 1))
{ setDate(7*N - 6 + (7+X-getDay())%7 ) }
return D }

and test with

for (Y=2000 ; Y<2030; Y++) {
document.writeln(NthXdayOfYM(0, 1, Y, 6), "<br>") }

shows Mondays in May 25-31 as needed.

I rather believe, without remembering proving it, that such jobs always
require a conditional or a %7.

Does any nation have a Special Day on the last, last but one, etc.,
X-day of February each year?

It's a good idea to read the newsgroup c.l.j and its FAQ. See below.

--
(c) John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v6.05 IE 6
news:comp.lang.javascript FAQ <URL:http://www.jibbering.com/faq/index.html>.
<URL:http://www.merlyn.demon.co.uk/js-index.htmjscr maths, dates, sources.
<URL:http://www.merlyn.demon.co.uk/TP/BP/Delphi/jscr/&c, FAQ items, links.

[dan]

Dr J R Stockton wrote:

In comp.lang.javascript message <46*********************@news.qwest.net>
, Tue, 29 May 2007 12:16:57, da*@nospam.com posted:

>Lee wrote:

>>da*@nospam.com said:
Lee wrote:
da*@nospam.com said:
>Am i breaking any rules when I loop dates like
>>
>// Determine Memorial Day
>intFlag = 0;
>memDayHol = new Date (currentYear, 4, 31);
>while (intFlag == 0) {
> if (memDayHol.getDay() == 1) {intFlag =1;}
> else {memDayHol = memDayHol - 1;}
>}
>>
>I can find no docs that one can use the '+' operator directly on a
>date object, and it assumes adding/subtracting a day(s). Is this
>deprecated code?
It certainly seems safer to use:
memDayHol.setDate(memDayHol.getDate()-1);
>
but the whole algorithm is nonsense.
If you want the date of the Monday before (or on) May 31, you
can calculate that without looping:
>
memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);

Gives June 1 in 2009 & 2015.

>>>Assume Memorial Day is 5/31. Then your code will set memorial day
to 5/29. I think you want

memDayHol = new Date (currentYear, 4, 31);
memDayHol.setDate(memDayHol.getDate()- (memDayHol.getDay() - 1));

Gives Saturdays.

>>>Plus my code is not nonsense as it correctly calculates the Memorial day.

The original code, as posted, fails to run.

>Oops I thought I saw 'a - (b + 1)'. My mistake. But back to my
original question, is the '+' an undocumented way of doing day
incrementing/decrementing?

It is undocumented because assigning a number to a variable "holding" a
Date Object disconnects the Date Object and makes the variable hold a
Number.
AIUI, Memorial Day is the last Monday in May in America - elsewhere, it
may be different.

Such code should, if it is required to run in all years, be tested in
all types of year. For ordinary dates, that's 14 types - Leap or not,
starting on Mon-Sun. All types necessarily occur in any 28-year stretch
lacking a missing Leap Year.

So test in this sort of manner :-

for (Y=2000 ; Y<2030; Y++) {
memDayHol = new Date (Y, 4, 31);
memDayHol.setDate(memDayHol.getDate()-memDayHol.getDay()+1);
document.writeln(memDayHol, "<br>") }
The easiest way to get the last X-day of a month is to think of it as
the Zeroth X-day of the following month. There is then no need to know
the length of any month.

Reading the FAQ can get you to

function NthXdayOfYM(N, X, Y, M) { var D
// X = Sun=0..Sat=6 Sun=7.. , M=1..12, N=0 for last of prev mth.
with (D = new Date(Y, M-1, 1))
{ setDate(7*N - 6 + (7+X-getDay())%7 ) }
return D }

and test with

for (Y=2000 ; Y<2030; Y++) {
document.writeln(NthXdayOfYM(0, 1, Y, 6), "<br>") }

shows Mondays in May 25-31 as needed.

I rather believe, without remembering proving it, that such jobs always
require a conditional or a %7.

Does any nation have a Special Day on the last, last but one, etc.,
X-day of February each year?

It's a good idea to read the newsgroup c.l.j and its FAQ. See below.

I was going to point out that May 31 1998 =June 1st, but I had to take
off yesterday.

Just to fill everyone in the javascript code I originally posted is
running within a Hyperion Reporting system in which it works. But
apparently this is undocumented, and so I will change my 'subtract day'
code to the documented way in case the code is moved to another system
in future.

I prefer my looping structure as this not a "PRO" shop, and others need
to be able to easily understand what is happening, and we will be
running this code only in the USA (my famous last words...). And this
isn't controlling rocket ship and so on. But thanks for the interesting
code example.

[Dr J R Stockton]

In comp.lang.javascript message <f3*********@drn.newsguy.com>, Tue, 29
May 2007 12:43:23, Lee <RE**************@cox.netposted:

>
In both browsers that I've tried, subtracting 1 from a Date
object works as I would expect, which is not what you're
looking for. The result of subtracting 1 from
"new Date (2007, 4, 31)" is a Number, not a date: 1180594799999

No, it is 1180565999999 here.

Evidently you are 7 hours away from the Real Time, probably -8+1.

--
(c) John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v6.05 IE 6.
Web <URL:http://www.merlyn.demon.co.uk/- w. FAQish topics, links, acronyms
PAS EXE etc : <URL:http://www.merlyn.demon.co.uk/programs/- see 00index.htm
Dates - miscdate.htm moredate.htm js-dates.htm pas-time.htm critdate.htm etc.