Skip to main content
Bethemesh
GuideBest practices

Date and time calculations: a practical guide

Date differences, durations, business days, ISO weeks, recurrences and formats: choose the right date and time calculation.

Published 7 September 2026Reading : 2 minBy Bethemesh Team
Beginner
Show contents
  1. Date difference or time duration?
  2. Add or subtract from a date
  3. Business days, ranges and recurrences
  4. Locate a date in the calendar
  5. Formats, timestamps and relative time

Date calculations look simple until month ends, leap years, weekends or different formats become involved. Bethemesh’s Date & Time family separates these problems into focused tools so calendar arithmetic is not confused with plain duration arithmetic.

Date difference or time duration?

Use the date difference calculator to compare calendar dates in years, months, days and total days. For a work shift or clock interval, the time duration calculator is more appropriate and can handle a period crossing midnight.

The duration converter converts values between milliseconds, seconds, minutes, hours, days and weeks. Those are arithmetic conversions; they do not replace calendar calculations when real month lengths matter.

Add or subtract from a date

The date arithmetic calculator adds or subtracts days, months or years while keeping month-end cases valid. The days until date calculator measures a countdown or elapsed days, while the age calculator applies calendar logic to a birth date and reference date.

Business days, ranges and recurrences

Calendar days and business days answer different questions. The business days calculator excludes weekends and can shift a date by a number of working days. The date range generator builds dates between two boundaries, while the recurring dates generator creates a sequence at a chosen frequency.

These tools are useful for schedules, deadlines and recurring plans without building spreadsheet formulas.

Locate a date in the calendar

The ISO week calculator returns the week number and ISO week-year, which can differ from the calendar year around New Year. The day of week calculator identifies the weekday, while the day of year calculator returns its position from 1 to 365 or 366.

Formats, timestamps and relative time

The date format converter presents the same date in several readable formats. The Unix timestamp converter works with a numeric representation of an instant and includes ISO 8601 output. The relative time calculator turns an interval into a human-oriented expression such as “in 3 days” or “2 hours ago”.

All these calculations run locally in your browser. Your dates do not need to be uploaded to Bethemesh.

For international scheduling, continue with the guide to time zones, daylight saving time and meetings.

Related tools

Dates & time

Age calculator

Calculate an exact age from a date of birth and a reference date.

100% local
Use this tool
Dates & time

Date difference calculator

Calculate the exact difference between two dates in days, weeks, months, or years.

100% local
Use this tool
Dates & time
Beta

Days Until a Date Calculator

Calculate how many days remain until a date, or how many days have passed.

100% localNew
Use this tool
Dates & time
Beta

Business Days Calculator

Count business days between two dates, excluding Saturdays and Sundays.

100% localNew
Use this tool
Dates & time
Beta

Duration Converter

Convert seconds, minutes, hours, days and weeks.

100% localNew
Use this tool
Dates & time
Beta

Time Duration Calculator

Calculate the duration between a start time and an end time.

100% localNew
Use this tool
Dates & time
Beta

ISO Week Number Calculator

Find the ISO week number of a date and the start and end dates of that week.

100% localNew
Use this tool
Dates & time

Timestamp converter

Convert a Unix timestamp to a readable date or turn a date into a Unix timestamp.

100% local
Use this tool
Dates & time

Date Format Converter

Convert dates between ISO, day-first and month-first formats.

100% localNew
Use this tool
Dates & time

Date Range Generator

Generate a list of dates between two endpoints.

100% localNew
Use this tool
Dates & time

Relative Time Calculator

Express a difference as “3 days ago” or “in 2 hours”.

100% localNew
Use this tool

Collection

Master dates and time

  1. 01Date and time calculations: a practical guide
  2. 02Time zones and meetings: avoid offset mistakes
  3. 03How to calculate work hours with a break
  4. 04How to calculate business hours between two dates and times
  5. 05Date formats: DD/MM/YYYY, MM/DD/YYYY and ISO 8601
  6. 06Create date lists and recurring dates for a schedule

Was this article useful?