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I am trying to compare two dates in javascript without the time portions. The first date comes from a jquery datepicker and the second date comes from a string.

Although I could swear that my method worked a while ago it looks like it is not working now.

My browser is Firefox but I also need my code to work in IE.

function selectedDateRetentionDaysElapsed() { var dateSelectedDate = $.datepicker.parseDate('dd/mm/yy', $('#selectedDate').val()); // dateSelectedDate is equal to date 2015-09-30T14:00:00.000Z var parsedRefDate = isoStringToDate('2015-11-10T00:00:00'); var reportingDate = getPredfinedDateWithoutTime(parsedRefDate); // reportingDate is equal to date 2015-11-10T13:00:00.000Z var businessDayCountFromCurrentReportingDate = getBusinessDayCount(dateSelectedDate,reportingDate); // businessDayCountFromCurrentReportingDate is equal to 39.9583333333336 if (businessDayCountFromCurrentReportingDate >= 40) { return true; } return false; } function isoStringToDate(dateStr) { var str = dateStr.split("T"); var date = str[0].split("-"); var time = str[1].split(":"); //constructor is new Date(year, month[, day[, hour[, minutes[, seconds[, milliseconds]]]]]); return new Date(date[0], date[1]-1, date[2], time[0], time[1], time[2], 0); } function getPredfinedDateWithoutTime(myDate) { myDate.setHours(0,0,0,0) return myDate; }

My issues are...

  1. My isoStringToDate function is returning a date with a time even though I am not specifying a time.
  2. The setHours call on a date does not seem to be working either.

Can someone please help me with this.

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  • 1. You are specifying time. You parse a string, extract values to time array and then pass it to the constructor. Commented Nov 11, 2015 at 5:44
  • There is no way that '2015-11-10T00:00:00' converted to a Date object will be the equivalent of '2015-09-30T13:00:00.000Z', those dates are 41 days apart. Also, the isoStringToDate function zeros the local hours, so unless you are in the GMT timezone, the UTC (i.e. Z) time zone, the time component will not be 00:00:00. Commented Nov 11, 2015 at 5:53
  • Hi Yelda. That's true but the time values are 00:00:00. So isn't that effectively a date without a time? Commented Nov 11, 2015 at 6:45
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    It's still not what you think. isoStringToDate('2015-11-10T00:00:00') will return a Date object for the equivalent local time of 00:00:00 on that date (no need to further zero the hours), but the actual time value will be UTC, so its hours won't be zero unless the host system is set to UTC. So if dateSelectedDate really is 00:00:00UTC, then there will always be a time difference unless your system is also set to UTC. You need both to be 00:00:00 in the same timezone, preferably UTC so daylight saving boundaries are avoided too. Commented Nov 11, 2015 at 6:50

1 Answer 1

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The simplest way to get the number of whole days between two dates is to create two date objects for the subject dates that are set to the same time. Noon is convenient as it means the date part is unaffected by daylight saving (some places introduce it at midnight) if you happen to print out just the date part.

The following does all calculations in the time zone of the host system. UTC could be used (and the hours set to 0 as daylight saving isn't an issue at all), but it's more to type.

E.g.:

function differenceInDays(d0, d1) { // Copy dates so don't affect originals d0 = new Date(+d0); d1 = new Date(+d1); // Set to noon d0.setHours(12,0,0,0); d1.setHours(12,0,0,0); // Get difference in whole days, divide by milliseconds in one day // and round to remove any daylight saving boundary effects return Math.round((d1-d0) / 8.64e7) } // Difference between 2015-11-12T17:35:32.124 and 2015-12-01T07:15:54.999 document.write(differenceInDays(new Date(2015,10,12,17,35,32,124), new Date(2015,11,01,07,15,54,999)));

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2 Comments

Thanks for that. It's a nice function. I am a bit confused where you are using the plus operator for the date copy. From what I have read I think you are converting the input dates to numbers using the plus operator but when I have a look at the js docs I can't see a new Date constructor for a number
@Richie—until ES5, if a Date object was passed as the sole value to the Date constructor, it was first converted to a string then parsed back into a Date object. Some browsers incorrectly parsed the string (believe it or not) in some circumstances, so it was safer to convert the Date to a number using +date, thereby passing the time value and ensuring the copy was accurate. However, since ES5, the Date value is converted to a time value instead (effectively the same as +date, see Date(value)).

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