DateTimeMate

DateTimeMate is a focused calculator for date and time math. Where a normal calculator handles numbers, this one handles moments and durations, so it can tell you the exact gap between two timestamps, convert one bundle of time units into another, or add a stretch of time to a starting date and give you the result. The site at datetimemate.com is the web version of the tool, and there's an open-source command-line version behind it that shares the same logic.

The core job is computing differences. Give it two dates or two times and it returns the span between them in a compact duration notation, using uppercase letters for the date parts, so years, months, weeks, and days, and lowercase for the time parts, so hours, minutes, seconds, and smaller. A result comes back as a single tidy string, something along the lines of six weeks, six days, nine hours, forty-four minutes, and forty-six seconds compressed onto one line, which is far easier to scan than a sprawled-out sentence.

It's flexible about what you feed it. Inputs can be ISO-8601 dates with timezone offsets, Unix timestamps, plain compact integers, or human shorthand like today and tomorrow, so you rarely have to reshape your data before pasting it in. That tolerance for messy input is part of what makes it quick to reach for, since you can drop in whatever format your logs or your calendar happen to use and let the tool sort it out instead of writing a one-off script to parse it first.

From there it does the related tasks you'd expect once you can measure time properly. It converts a duration from one set of units to another, for example turning a large pile of seconds into weeks, days, hours, minutes, and seconds. It adds a duration to a starting date and can repeat that step until it reaches an end date, which is handy for working out a recurring schedule or a series of deadlines. It also reformats a date or time from one layout into another, and it adds and subtracts two separate durations when you need to combine or difference spans of time rather than dates.

Precision and timezones are where it earns its keep. The notation goes all the way down to milliseconds, microseconds, and nanoseconds, and there's a brief output mode when you want the terse version rather than the spelled-out one. It also converts between timezones with daylight-saving-time awareness, so an answer that crosses a clock change stays correct instead of quietly drifting by an hour. That level of care is aimed at people who actually need the exact answer, so developers, sysadmins, and data folks scripting around log timestamps or schedules will feel more at home here than they would in a friendly consumer date widget.

The kinds of questions it answers come up more often than you'd think. How long ago did this log line fire, how many days until a contract renews, what's the total once you add a two-week sprint to a start date and repeat it through the quarter, or what a UTC timestamp looks like in another timezone once the clocks have shifted. Each of those is a small annoyance to work out by hand and easy to get wrong, and DateTimeMate turns them into a single query with an exact answer. The same goes for the reverse direction, taking a duration you already have and reshaping it into whatever units make sense to report, so a raw count of seconds becomes a readable span you can drop into a status update or a log line.

What sets it apart from the many generic date calculators online is the combination of format flexibility, exact notation, and an open-source engine. DateTimeMate started as a Go package and command-line tool released under the MIT license, so the same logic that powers the website can be installed and run locally, dropped into your own scripts, or self-hosted. You can pull it in through Go, install it with Homebrew on macOS or Linux, or grab a prebuilt binary from the project's releases, which makes it as comfortable inside an automated pipeline as it is in a browser tab. Because it's a package as well as a CLI, you can also call it directly from your own Go programs when you need the same date math in code.

Access is simple. The website is free to use in the browser, and the command-line tool is free and open source on GitHub. It's a narrow tool by design, so it won't manage your calendar, send reminders, or replace a full scheduling app, and the compact notation takes a moment to get used to. But for the specific job of doing date and time arithmetic accurately and scriptably, it's a sharp, no-friction option that does one thing and does it precisely. If you've ever opened a spreadsheet just to work out a date difference, or written the same throwaway parsing script more than once, this is the sort of small utility that quietly removes that chore for good.