These guides show how to work out date and time math yourself, by hand or in a spreadsheet, with worked examples you can check. Each one pairs with a DateTimeCalcs calculator that gives the same answer instantly.
A calculator gives you the number. A guide is for when you need to know why it is that number. That might mean checking a payroll total before you sign it, building a time sheet in Excel or settling whether 2100 is a leap year. It also helps when explaining to a colleague why June 1 to June 5 can be 4 days or 5.
Each guide sets out the method and works through real examples. Most also give the spreadsheet formulas that do the same job and the mistakes that most often produce a wrong answer, such as typing 7 hours 30 minutes as 7.30.
Where the rules come from
Work-time rules, such as which breaks count as hours worked and how punches may be rounded, come from the Department of Labor’s regulations in Title 29 of the Code of Federal Regulations. State laws and employer policies can add their own. Calendar rules come from the National Institute of Standards and Technology (NIST) and NASA, and court deadlines from the Federal Rules of Civil Procedure. Every guide lists its sources with the date they were checked.
For a quick answer instead, the time duration calculator finds the hours and minutes between two times, including overnight shifts and decimal hours.