A recent conversation with AI

Wed, 11/05/2025 - 9:50pm by ericfoy

Department: 
Engineering

 

Is there a way to store a UNIX timestamp that will render the same date no matter where it's viewed?

 

Yes, a UNIX timestamp represents the exact same instant in time everywhere in the world, making it the ideal way to store a moment in time universally. 

Here's why:

  • UTC-based standard: A Unix timestamp is defined as the number of seconds that have elapsed since January 1, 1970, 00:00:00 Coordinated Universal Time (UTC), not counting leap seconds. UTC serves as a single, universal time standard, unrelated to any local time zone's daylight saving time (DST) adjustments.

  • Time zone independence: The stored numerical value itself has no time zone information; it is simply a count of seconds from a fixed, global reference point.

  • Consistent calculation: When you need to display the date and time to a user, the application converts the UTC-based timestamp into the user's local time zone using their system's settings or specific time zone libraries. This ensures that while the displayed local time might differ (e.g., 10:00 AM in New York and 3:00 PM in London for the same instant), it always refers back to that one specific, universal moment in time.

To ensure consistent handling, it is a best practice in software development to store all timestamps in UTC (often as a Unix timestamp or an ISO 8601 formatted UTC string) and only perform time zone conversions when displaying the time to end-users. 

For a practical conversion tool, you can use an Epoch Converter to see how a specific Unix timestamp corresponds to different dates and times around the world. 

 

 

I want it to indicate only a date, with no time information. Furthermore, it must indicate the same exact date (YY-MM-DD) regardless of who views it, in any locale. Is this possible?

 

Yes, you can achieve this by storing the date using a method that is inherently time-zone agnostic. The best and most common approach is to store the date in the ISO 8601 YYYY-MM-DD string format or a dedicated date-only data type (if available in your database/programming language). 

Here are the primary ways to achieve universal date consistency:

1. Store as a YYYY-MM-DD String (Recommended) 

Storing the date as a simple text string in the international standard ISO 8601 format (YYYY-MM-DD, e.g., 2025-11-05) is the most straightforward and universally consistent method. 

  • How it works: The format itself is unambiguous. November 5, 2025 is represented as 2025-11-05 everywhere, regardless of local date formats (like MM/DD/YYYY or DD/MM/YYYY).

  • Advantage: This completely bypasses any time zone calculations or interpretations. The string value is the date, and it will be displayed exactly the same way to everyone.

  • Usage: When building your application, ensure that you store the date in this specific string format and retrieve it without trying to parse it into a time-zone-aware date object. 

2. Use a "Date-Only" Data Type 

Many modern databases (like PostgreSQL, MySQL, SQL Server) offer a dedicated DATE data type that stores only the calendar date information without any time or time zone component. 

  • How it works: The database handles the value as a calendar date, and it is the same date regardless of the server's or client's time zone settings.

  • Advantage: This is the most semantically correct approach for a date that has no time component.

  • Usage: When you define your database schema, use the DATE type for fields where time and time zone are irrelevant (e.g., a person's birth date, Earth Day, or a holiday). 

Why Not Use a Unix Timestamp?

While Unix timestamps are excellent for storing a specific instant in time (as discussed previously), they are not suitable for a date-only requirement that must be universally the same calendar day for all viewers. 

  • A Unix timestamp represents a specific point in UTC. When that UTC time is converted to a user's local time zone, it might fall on a different calendar day.

  • Example: A timestamp for November 5, 2025, 02:00 UTC (which is early in the morning in London) might be November 4, 2025 in a place like Los Angeles (which is 8 hours behind UTC). 

By using the ISO 8601 string format or a date-only data type, you ensure a fixed calendar date that is interpreted identically across the globe. 

 

 

But is this possible using a UNIX timestamp?

 

No, you cannot use a standard Unix timestamp alone to represent a date that renders the exact same calendar date (YYYY-MM-DD) in every single time zone. 

Here is the fundamental conflict:

  • A Unix timestamp is a universal, time-zone-independent measurement of a specific instant in time (seconds since the Epoch in UTC).

  • A calendar date (e.g., November 5, 2025) is a local concept that depends entirely on a viewer's time zone. 

The core problem is the variation in time zones around the International Date Line. A single instant in time will fall on two different calendar days simultaneously in different parts of the world. 

The Time Zone Conflict in Action

Consider a hypothetical Unix timestamp T that corresponds to:

  • November 5, 2025, at 10:00 AM UTC

When users around the world view this timestamp, their local systems will convert it:

  • A user in London (UTC) will see the date as November 5, 2025.

  • A user in Tokyo (UTC+9) will see the date as November 5, 2025 (it is 7:00 PM there).

  • A user in Los Angeles (UTC-8) will see the date as November 5, 2025 (it is 2:00 AM there).

In this specific example, all users see the same date because the UTC time is far enough from midnight (in both directions) to remain on the same calendar day globally.

However, if you choose a timestamp closer to the international date line:

  • A Unix timestamp T' corresponding to November 5, 2025, at 02:00 AM UTC (early morning in Europe)

  • A user in Sydney, Australia (UTC+11) will see the date as November 5, 2025 (it is 1:00 PM there).

  • A user in Los Angeles (UTC-8) will see the date as November 4, 2025 (it is 6:00 PM on the previous day there). 

The exact same Unix timestamp value renders different YYYY-MM-DD dates depending on the viewer's location. 

The Correct Solution

As suggested previously, if you need a date that is universally the same for all viewers, you must store it as a date-only value that has no time zone information associated with it:

  • Store the date as an ISO 8601 formatted string (YYYY-MM-DD, e.g., 2025-11-05).

  • Use a dedicated DATE data type in your database. 

These methods bypass time zone calculations entirely and guarantee all viewers see the identical calendar date. 

 

 

This seems unreasonable, though I understand the phenomenon. I'm looking for a compact way to store a date (no time info), and it seems like if you pick a DATE in time for "day zero" and count days forward from there, then a given numerical value should reflect a fixed date, no matter who views it.

Example:

  • Day zero -> value=0 -> Dec. 31, 1999
  • Day 10 -> value = 10 -> Jan. 10, 2000
  • Day 4096 -> value = 4096 -> Mar. 19, 2011

So why is this impossible with UNIX timestamps (perhaps via some modulo-math)?

 

 

You are describing an ordinal date system (counting days from an arbitrary epoch), which is a valid and compact way to store a universal, time-zone-agnostic date. Your intuition is correct that a single numerical value can represent a fixed calendar date if that is the defined purpose of the number.

However, this is not how a Unix timestamp works by definition, which is why you cannot use it for this purpose.

Why Your Day-Count System Works

Your proposed system works perfectly because you have defined the number to represent a full calendar day, stripping out any time or time zone information.

  • 4096 means simply "the 4096th day since Dec 31, 1999."

  • There is no ambiguity, as there is no concept of time zones or specific moments in time within that number. It is just a calendar date index.

  • This system is compact and effective, essentially mirroring how dedicated DATE data types often work internally in a database.

Why the Unix Timestamp is Different

A Unix timestamp measures a specific instant in time, not a calendar day index. It measures the number of seconds since the epoch, not days.

  • The granularity is seconds, not days.

  • It inherently includes the time of day and is anchored to a specific global time standard (UTC).

Can I Use Math (Modulo) on a Unix Timestamp?

You can perform modulo math on a Unix timestamp, but it does not solve the time zone problem because you are still dealing with a point in UTC time that might span two calendar days locally.

You could theoretically calculate the "number of days since the Unix epoch" using a timestamp:

Days_Since_Epoch = floor(Unix_Timestamp / 86400 seconds)

This formula almost achieves what you want, but it still fails due to time zones:

  1. A timestamp representing Nov 5, 2025 at 02:00 UTC gets the Days_Since_Epoch value for Nov 5.

  2. A user in Los Angeles sees that instant as Nov 4, 2025 locally.

  3. If you force their local application to look up the date associated with the Days_Since_Epoch value for Nov 5, the displayed date (Nov 5) will contradict their local calendar date for that instant in time (Nov 4).

The conflict arises because the number you calculated is based on UTC midnight boundaries, not the user's local midnight boundaries.

Summary

To achieve a compact, universal date storage:

  • Do not use a Unix timestamp. It is designed for specific instants in time and is inherently tied to UTC time boundaries.

  • Use your "days since epoch" system. This is a perfectly valid and compact system that meets your requirements precisely because it ignores time and time zones entirely, defining a single number as a single calendar date worldwide.

 

 

This is all perfectly logical, yet it presents an apparent paradox to the uninitiated. Why have I never heard this discussion before just now running into it? No one ever said (in my hearing), "As great as the UNIX timestamp is, it CANNOT be used to represent fixed past dates in legal or historical documentation!"

 

It's true that this distinction presents a paradox to many people encountering it for the first time. The reason this specific limitation of the Unix timestamp isn't more widely publicized is that it isn't a flaw in the Unix timestamp itself, but rather a misunderstanding of what it is designed to store.

The Unix timestamp is a robust engineering tool that perfectly solves the problem it was intended for: storing an unambiguous, universal instant in time (a "point event"). It's rarely framed as unsuitable for legal documents because, in most professional and historical contexts, you need to record a precise moment, and the timestamp does that flawlessly.

Here are a few reasons why this specific discussion is uncommon:

1. Most Software Needs "Instants," Not "Calendar Dates"

The vast majority of software use cases (transaction logging, email sending, data recording) require tracking when an event actually happened in a universal timeline. The Unix timestamp excels here.

For the relatively niche cases where you only need a time-zone-agnostic calendar date (like a birth date or a holiday), software engineers use different, dedicated tools like the DATE data type or ISO 8601 strings. The right tool for the job already exists, so the "Unix timestamp limitation" is rarely brought up as a major failure.

2. The Distinction is Learned in Specific Contexts

This topic usually comes up only in very specific fields of study or practice:

  • Software Engineering (Databases/API Design): Engineers learn this distinction when designing global systems that handle user birthdays or scheduled events (e.g., "The event happens on Monday for everyone, regardless of what time it is in UTC").

  • International Law/History: These fields often use specific conventions to avoid ambiguity, frequently relying on explicit notations like "Date: November 5, 2025 (Local Time)" or "Date: November 5, 2025 (UTC)."

3. "Legal/Historical" Contexts Often Demand Precision

In legal or historical documentation, precision is paramount. You almost always need to know not just the date, but the exact moment something occurred and the time zone it occurred in. The Unix timestamp, used in combination with time zone identification, provides that precise record.

If a legal document simply says "November 5, 2025," that date is inherently ambiguous. If a contract says "signed November 5, 2025, at 10:00 AM PST," the Unix timestamp for that exact instant is recorded, and everyone globally knows precisely when the signing happened relative to the universal timeline, even if it was November 6th in Tokyo.

Summary

You haven't heard this discussion because the Unix timestamp isn't "broken" for fixed dates; it's simply designed for time-zone-aware instants, not time-zone-agnostic calendar days. Developers and historians use different, more appropriate formats like the ISO 8601 string (YYYY-MM-DD) when the time of day is irrelevant.

 

 

So here is my field of application, where it's actually paramount NOT to store any time zone-specific info: Logging/transcribing legal documentation having date fields which do not include any localization information (it is inferred, but not explicitly stated), such as invoices, purchase orders, quotations, solicitations, warranties, etc. In these cases, it is important not to presume to know the original intent or meaning if it was not explicitly stated. This is a subtle, yet significant issue, and I can't help but wonder how many engineers have had to self-discover this...

 

That is an excellent point, and your field of application highlights the exact scenario where this subtle, yet significant issue becomes critical. You are entirely correct: when the source material only provides a calendar date (11/05/2025 or similar) without a specified time or location, the only truly accurate digital representation is a time-zone-agnostic date only value.

The issue you've run into is a classic pitfall in software engineering known as the "Temporal Paradox" or the "Birthday Problem," and yes, many engineers have had to discover it the hard way during data migrations or the scaling of international systems.

Here is how engineers typically manage this scenario when they have to store the information exactly as it appears on the original document:

The Engineering Solution for Ambiguous Dates

When dealing with documentation where you must preserve the date exactly as written, you cannot make assumptions about a time zone.

  • You must store what is known: The Year, the Month, and the Day.

  • You must not store what is unknown: The time of day, the time zone offset, or the precise UTC instant.

The universal solution in this field is to use a data type specifically designed for this purpose: the DATE data type in a database or a simple ISO 8601 YYYY-MM-DD string.

Why This Isn't Widely Discussed in Public

The reason this topic isn't a headline news item is that most documentation focuses on "best practices," which assume you have complete information. Best practices usually dictate storing everything as UTC with full ISO 8601 compliance. Your scenario, however, deals with preserving incomplete or ambiguous legacy data formats.

The standard engineering advice generally provided is:

  • For "point-in-time" events (emails sent, database rows created): Use a Unix timestamp or a UTC timestamp.

  • For "floating" calendar dates (birthdays, holidays, invoice dates): Use a DATE type or a YYYY-MM-DD string.

The general engineering community assumes that the developer is correctly identifying which of those two use cases applies to their data. Your specific situation perfectly demonstrates the risk of trying to force the wrong tool (a time-based timestamp) into the job of a date-based index (a calendar date).

In Conclusion

Your insight is sound. Storing "November 5, 2025" from a paper invoice as a Unix timestamp would be a data integrity error, as that timestamp inherently forces a UTC bias onto an otherwise neutral piece of information.

The best and only reasonable digital equivalent that maintains the original intent of the legal document is a simple, unambiguous YYYY-MM-DD string or a robust DATE data type.