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Javascript – Need help to understand Date rollover logic

I am trying to understand the logic to rollback the date to previous month.

const now = new Date();
console.log(`Initial Date: ${now}`); // Initial Date: Wed Oct 23 2024 00:01:56 GMT-0500 (CST)

now.setDate(-1);
console.log(`Updated Date: ${now}`); // Updated Date: Sun Sep 29 2024 00:01:56 GMT-0500 (CST)

Here, why the Updated Date in not showing as Sep 30?

I am unable to understand the output Updated Date: Sun Sep 29 2024 00:01:56 GMT-0500 (CST)

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3

Answers


  1. 0

    When you use now.setDate(-1), you’re not actually rolling back the date to the previous month. Instead, you’re setting the date to the 1st day of the current month minus 1 day.

    Here’s how it works:

    1. Initial Date: Let’s say today is October 23, 2024.
    2. Setting the Date: When you call now.setDate(-1), you’re telling it
      to set the date to the 1st of October (because the Date object
      rolls back to the start of the month) and then subtract 1 day,
      which results in September 29, 2024.

    Use this to get the

    const now = new Date();
const lastDayOfPreviousMonth = new Date(now.getFullYear(), now.getMonth(), 0);

console.log(`Last Day of Previous Month: ${lastDayOfPreviousMonth}`);

    Explanation:

    • new Date(year, monthIndex, day): This constructor creates a new date.
    • The monthIndex is zero-based (0 for January, 1 for February, etc.).
    • now.getFullYear(): Gets the current year. now.getMonth() – 1: Gets he
      previous month.
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  2. 0

    The behavior you are encountering is due to how JavaScript’s Date object handles negative values for dates. Let’s break it down step-by-step.

    Why now.setDate(-1) results in Sep 29 instead of Sep 30

    1.Understanding setDate() behavior:

    When you pass 0 to setDate(), it sets the date to the last day of the previous month.
    If you pass a negative value, JavaScript rolls back into the previous month, counting backwards from the last day.

    2.In your case:

    The initial date is Oct 23, 2024.
    When you call now.setDate(-1), it doesn’t just jump to the last day of the previous month (Sep 30). Instead:
    Date -1 means one day before Sep 1.
    So, it keeps rolling backwards to the end of the previous month: August 31, and subtracts 1 more day to land on Sep 29.

    3.Expected Output with setDate(0): If you want to get Sep 30 (the last day of September), use:

    now.setDate(0);
console.log(`Updated Date: ${now}`);

    setDate(0) will take the date back to Sep 30 (since 2024 is a leap year and September has 30 days).
    Key Takeaways:
    setDate(-1) rolls back across months in an unexpected way because it counts backwards continuously from the current month into previous months.
    Use setDate(0) if you want the last day of the previous month.
    This behavior is a quirk of JavaScript’s Date object when handling negative values for dates.

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  3. 0

    You can find an answer in the official documentation:
    https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/setDate

    The setDate() method of Date instances changes the day of the month
    for this date according to local time.

    • setDate() function does not execute relative calculation to the date stored in variable. But rather sets the actual date based on value providedl
    • Treat the value of setDate() function as index of an array with
      dates, rather than actual day you want to subtract or add

    [29 sept, 30 sept, 1 oct, …. 30 oct, 31 oct, 1 nov]

    [-1,0,1,…30,31, 32]

    setting date to 0 will always point you to the last day of the previous month:

    const date = new Date();
// 23 oct
console.log(date.toDateString());
date.setDate(0);
// 30 sep
console.log(date.toDateString());

// important!: you have changed the month to september with setDate(0)
//so you will be looking up for an array of dates in september now
date.setDate(1);
// 1 sep
console.log(date.toDateString());

    So in your example setting it to -1 will return 29th of September.

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