How To Make Excel Calculate Days Between Dates

Calculate days between dates in Excel with precision. Enter your dates below to see the formula and results.

Complete Guide: How to Make Excel Calculate Days Between Dates

Calculating the difference between dates is one of the most common tasks in Excel, whether you’re tracking project timelines, calculating employee tenure, or analyzing financial periods. This comprehensive guide will teach you everything you need to know about date calculations in Excel, from basic formulas to advanced techniques.

Understanding Excel Date Serial Numbers

Before diving into calculations, it’s crucial to understand how Excel stores dates. Excel doesn’t store dates as text or in a special date format – instead, it uses a serial number system where:

• January 1, 1900 is stored as serial number 1
• January 2, 1900 is stored as serial number 2
• Each subsequent day increments by 1

This system allows Excel to perform mathematical operations on dates, which is what enables date difference calculations.

Basic Date Difference Formula

The simplest way to calculate days between dates is to subtract one date from another:

=End_Date - Start_Date

For example, if cell A1 contains 1/15/2023 and cell B1 contains 1/30/2023, the formula =B1-A1 would return 15.

Using the DATEDIF Function

For more control over date calculations, Excel provides the DATEDIF function (Date DIFFerence). This function can calculate differences in days, months, or years:

=DATEDIF(start_date, end_date, unit)

The unit parameter accepts:

• "d" – Days
• "m" – Complete months
• "y" – Complete years
• "ym" – Months excluding years
• "yd" – Days excluding years
• "md" – Days excluding months and years

Official Microsoft Documentation

For complete technical specifications of Excel’s date functions, refer to the official Microsoft DATEDIF documentation.

Advanced Date Calculations

For more complex scenarios, you can combine date functions:

Calculating Weekdays Only

To count only business days (excluding weekends):

=NETWORKDAYS(start_date, end_date)

To exclude specific holidays:

=NETWORKDAYS(start_date, end_date, holidays)

Where “holidays” is a range containing holiday dates.

Calculating Age

For precise age calculations that account for whether the birthday has occurred this year:

=DATEDIF(birth_date, TODAY(), "y") & " years, " & DATEDIF(birth_date, TODAY(), "ym") & " months, " & DATEDIF(birth_date, TODAY(), "md") & " days"

Common Errors and Solutions

Error	Cause	Solution
#VALUE!	Non-date values in calculation	Ensure both cells contain valid dates or use DATEVALUE() to convert text to dates
Negative number	End date is before start date	Swap the dates or use ABS() function to get absolute value
######	Column too narrow to display result	Widen the column or change number format
Incorrect month count	DATEDIF counts complete months only	Use combination of “y” and “ym” for more accurate month counts

Date Formatting Tips

Proper date formatting ensures your calculations work correctly:

1. Select the cells containing dates
2. Press Ctrl+1 (Windows) or Command+1 (Mac) to open Format Cells
3. Choose the Date category
4. Select your preferred date format (e.g., 3/14/2023 or 14-Mar-2023)
5. Click OK to apply

For international date formats, you may need to adjust your system’s regional settings or use custom formats like dd/mm/yyyy.

Performance Considerations

When working with large datasets containing date calculations:

• Use helper columns for complex calculations rather than nested functions
• Consider using Power Query for date transformations on large datasets
• For dashboards, pre-calculate date differences rather than using volatile functions like TODAY()
• Use Table references instead of cell ranges for better maintainability

Excel vs. Google Sheets Date Functions

While Excel and Google Sheets share many similar functions, there are some key differences in date calculations:

Feature	Excel	Google Sheets
Date Serial Number Start	January 1, 1900 = 1	December 30, 1899 = 1
DATEDIF Function	Available (undocumented)	Available (documented)
NETWORKDAYS	Available	Available as NETWORKDAYS
WORKDAY Function	Available as WORKDAY.INTL	Available as WORKDAY
Date Format Recognition	Strict format requirements	More flexible format recognition
Array Formulas for Dates	Requires Ctrl+Shift+Enter (pre-365)	Automatic array handling

Academic Research on Date Calculations

The National Institute of Standards and Technology (NIST) provides comprehensive guidelines on date and time representations in computational systems, which can help understand the foundations of Excel’s date handling.

Practical Applications of Date Calculations

Project Management

Date calculations are essential for:

• Creating Gantt charts and timelines
• Calculating project durations
• Tracking milestones and deadlines
• Resource allocation planning

Human Resources

HR departments use date calculations for:

• Employee tenure calculations
• Vacation and sick leave accrual
• Probation period tracking
• Benefits eligibility determination

Financial Analysis

Financial professionals rely on date calculations for:

• Interest calculations (daily, monthly, annual)
• Loan amortization schedules
• Investment holding periods
• Fiscal year reporting
• Depreciation schedules

Inventory Management

Date calculations help with:

• Shelf life tracking for perishable goods
• Stock rotation management
• Lead time analysis
• Seasonal demand forecasting

Automating Date Calculations with VBA

For repetitive date calculations, you can create custom functions using VBA (Visual Basic for Applications):

Function DaysBetween(startDate As Date, endDate As Date, Optional includeEnd As Boolean = False) As Long
    If includeEnd Then
        DaysBetween = endDate - startDate + 1
    Else
        DaysBetween = endDate - startDate
    End If
End Function
        

To use this function:

1. Press Alt+F11 to open the VBA editor
2. Insert a new module (Insert > Module)
3. Paste the code above
4. Close the editor and use =DaysBetween(A1,B1,TRUE) in your worksheet

Best Practices for Date Calculations

1. Always validate your dates: Use ISNUMBER() to check if a cell contains a valid date before calculations
2. Document your formulas: Add comments explaining complex date calculations
3. Use named ranges: For frequently used date ranges (e.g., “ProjectStart”, “ProjectEnd”)
4. Consider time zones: If working with international dates, use UTC or specify time zones
5. Test edge cases: Verify calculations with dates at month/year boundaries
6. Use consistent formats: Standardize date formats across your workbook
7. Backup your work: Date calculations can be sensitive to format changes

Government Time Standards

The NIST Time and Frequency Division provides official time measurement standards that underpin how computers (including Excel) handle date and time calculations.