如果我以 UTC +0 保存所有日期，如何在不同时区获得一致的数据

Question

I have a system where users from Washington DC can create a post . This post is saved in my system in UTC +0 time. Then, I can use a reporting system which will give me info about every created post in a certain date range. Lets say I select a date range from March 21st 00:00:00 to March 28th 23:59:59 but in my system someone created a post on March 28th 22:30:00 Washington DC time. Washington DC is several hours behind UTC, so this post would be saved at around March 29th 02:30:00 , and so when I generate the report for March 21st to March 28th, I will not get the correct result because there is 1 post that has been created on March 28th Washington time, but that is March 29th UTC +0 time.

I first solved this by obtaining the UTC offset of the client and sending it to the server, and so adding that offset to my date range:

// JavaScript
"offsetHours" : parseInt(new Date().getTimezoneOffset() / -60)
"offsetMinutes" : (new Date().getTimezoneOffset() / -60) % 1 * 60
// Python
_range["from"] = strToDate(_range["from"]) - datetime.timedelta(hours = int(request.headers["offsetHours"]), minutes=int(request.headers["offsetMinutes"]))

This solved the issue, but it raised another. Now if I generate a report for the same time range (from 21st to 28th March) from 2 different timezones, I will get different results. This is due to the fact that the 2 users have different offsets and so they affect the from range in different intervals.

Is there any solution to this problem?

Answer 1

You're not necessarily describing a problem , but rather a side effect of how local times around the world work.

At any given time, there is usually more than one "date" in effect somewhere in the world. If you are saving the timestamp of an event that took place, and you have customers around the world, you're not necessarily saving it with the same date that the user thought it was in their own time zone. This is true whether you align the timestamps to UTC or to a specific time zone.

Therefore, you must make a business decision about how your application is intended to work. Do you want your daily reports to reflect posts that were made within a UTC day, or within the day according to the time zone of your business's headquarters? Then store the timestamp in UTC and (optionally) adjust to your business's time zone before or during reporting.

If however you want the daily reports to reflect the date in the user's time zone, then you might want to also store the user's time zone ID (such as America/New_York - not a numeric offset) so that you could convert to that. Keep in mind that if user's are in different time zones, your reports might look strange when examined from a single time zone's perspective.

Another technique that is often used (primarily for performance reasons, but also for clarity of logic), is to keep both a UTC-based timestamp and a separate field for the "business date" that applies. Usually such a field is just a date field, storing a value such as 2021-03-29 without any time or time zone. This field can be pre-converted to a time zone according to whatever rules you decide are applicable for your business. It then becomes a great candidate for an index and works well for range queries for daily reports.

In the end - there is no one "right" way to do it. You have to decide what works best for your use case. If you are working for a larger company and unsure of the business requirements, then ask someone who might already perform a similar activity manually. (Often this is an accounting or sales person in a larger organization.)