Audit all usage of .toLocaleString.

[doms3]

In the issue XPN-67 an off-by-one issue was caused by time-zone inconsistencies. See the following explanation:

The off-by-one error in your code arises due to time zone discrepancies when parsing and formatting dates in JavaScript. Specifically, when you create a Date object using a date string (e.g., "2023-10-25"), JavaScript interprets it as midnight in UTC time. However, when you format this date using toLocaleString without specifying a time zone, it converts the time to your local time zone, which can result in the date shifting backward or forward depending on your location relative to UTC.

Here's a step-by-step explanation:

1. Parsing the Date String:
   • new Date(expense.date) takes a date string like "2023-10-25" and creates a Date object at 2023-10-25T00:00:00Z (midnight UTC).
2. Time Zone Conversion:
   • When you call toLocaleString("default", options) without specifying a time zone, it formats the date according to the local time zone of the environment running the code.
   • If your local time zone is behind UTC (e.g., UTC-5 for Eastern Time), the date shifts backward by several hours, potentially changing the day to the previous date.
3. Resulting Off-by-One Error:
   • The formatted date ends up being one day earlier than expected because the time zone conversion subtracts hours from the UTC time, pushing the time into the previous day in your local time zone.

Example:

// Suppose expense.date is "2023-10-25"
const dateString = "2023-10-25";
const date = new Date(dateString); // Interpreted as 2023-10-25T00:00:00Z
// If local time zone is UTC-5
// Local time: 2023-10-24T19:00:00-05:00
console.log(date.toLocaleString("default", {
  month: "short",
  day: "numeric",
  year: "numeric"
}));
// Outputs: "Oct 24, 2023" instead of "Oct 25, 2023"

Solution:

To fix the off-by-one error, you should ensure that the date is interpreted and formatted in the same time zone. There are a couple of ways to achieve this:

1. Specify the Time Zone in toLocaleString:

   console.log(new Date(expense.date)
    .toLocaleString("default", {
      month: "short",
      day: "numeric",
      year: "numeric",
      timeZone: "UTC" // Add this line
    })
    .replace(",", ""));

   By setting timeZone: "UTC", you ensure that the date is formatted in UTC time, matching how it was parsed.

2. Use toLocaleDateString Instead of toLocaleString:

   console.log(new Date(expense.date)
    .toLocaleDateString("default", {
      month: "short",
      day: "numeric",
      year: "numeric"
    })
    .replace(",", ""));

   The toLocaleDateString method focuses on the date part and may handle time zones differently, reducing the likelihood of an off-by-one error.

3. Parse the Date Components Manually:

   If expense.date is a string, you can split it into components and create a Date object using local time:

   const [year, month, day] = expense.date.split("-").map(Number);
   console.log(new Date(year, month - 1, day)
    .toLocaleString("default", {
      month: "short",
      day: "numeric",
      year: "numeric"
    })
    .replace(",", ""));

   This approach constructs the Date object using local time, avoiding the UTC conversion issue.

Summary:

The off-by-one error occurs because of time zone differences between parsing and formatting dates. By ensuring that both parsing and formatting occur in the same time zone (preferably your local time zone or UTC), you can eliminate the discrepancy.

In the end, I used Solution 1 to fix the issue but we should:

• Audit all usages of .toLocaleString to make sure the issue isn't present elsewhere.

• Decide if the fix I chose is the best one.

[linear[bot]]

XPN-68 Audit all usage of .toLocaleString.