How to calculate months between specific dates in Excel?

User Question

I’m managing a project timeline in Excel and need to calculate the duration in months between start and end dates. Some projects span multiple years, and I want to account for partial months. How can I accurately calculate this in Excel, both using formulas and VBA, especially when dealing with multiple projects?

Find duration between two dates in Excel 365

Method 1: Using Excel Formulas

Sample Data

We’ll use the following data in this tutorial examples:

ProjectID,ProjectName,StartDate,EndDate
1,Website Redesign,2024-01-15,2024-06-30
2,Mobile App Development,2024-03-01,2024-12-15
3,Data Migration,2024-02-10,2024-05-20
4,Cloud Infrastructure Setup,2024-04-01,2024-09-30
5,AI Implementation,2024-05-15,2025-03-31
6,Security Audit,2024-07-01,2024-10-31
7,ERP System Upgrade,2024-08-15,2025-02-28

Step-by-Step Instructions

  1. Open your Excel spreadsheet.
  2. Ensure you have two columns with your start and end dates.
  3. In a new column, enter the following formula:
=DATEDIF(C2, D2, "m") + (DAY(D2) - DAY(C2)) / DAY(D2)

Replace A2 with your start date cell and B2 with your end date cell.

  1. Press Enter to calculate the result.
  2. Format the cell as a number with 2 decimal places for accuracy.

Explanation of Key Concepts

  • The DATEDIF function calculates the difference between two dates in various units.
  • We use “m” as the third argument to get the difference in complete months.
  • The second part of the formula (DAY(D2) - DAY(C2)) / DAY(D2) accounts for partial months.
  • This formula works across year boundaries and accounts for varying month lengths.

Method 2: Using VBA

For multiple projects, using VBA can automate the calculation process.

VBA Code

First off, enable the Developer tab. Then use the code below to show the time duration in months using Excel VBA:

Sub CalculateProjectDurationsMonths()
    Dim ws As Worksheet
    Dim lastRow As Long
    Dim i As Long

    ' Set the active worksheet
    Set ws = ActiveSheet

    ' Find the last row with data
    lastRow = ws.Cells(ws.Rows.Count, "A").End(xlUp).Row

    ' Add header for the new column
    ws.Cells(1, 5).Value = "Duration (Months)"

    ' Loop through each row and calculate duration
    For i = 2 To lastRow
        Dim startDate As Date
        Dim endDate As Date
        Dim duration As Double

        startDate = ws.Cells(i, 3).Value
        endDate = ws.Cells(i, 4).Value

        ' Calculate duration using the formula
        duration = DateDiff("m", startDate, endDate) + _
                   (Day(endDate) - Day(startDate)) / Day(endDate)

        ' Write the result to the new column
        ws.Cells(i, 5).Value = Round(duration, 2)
    Next i

    ' Format the new column
    ws.Columns("E").NumberFormat = "0.00"
    ws.Columns("E").AutoFit

    MsgBox "Project durations have been calculated and added to column E.", vbInformation
End Sub

How to Use the VBA Code

  1. Open your Excel workbook and press Alt + F11 to open the VBA editor.
  2. Insert a new module and paste the above code into it.
  3. Close the VBA editor and return to your Excel sheet.
  4. Ensure your data is in the format shown in the CSV above, with columns A to D containing ProjectID, ProjectName, StartDate, and EndDate respectively.
  5. Run the macro by going to Developer > Macros, selecting “CalculateProjectDurationsMonths”, and clicking “Run”.

This VBA code will add a new column “Duration (Months)” and calculate the duration for each project, accounting for partial months.

Can’t calculate time duration in Excel

Common issues and solutions for both methods:

  • #NUM! error or “Type mismatch” error: Ensure your end date is later than your start date and that all dates are in a format Excel recognizes.
  • Incorrect results: Check that your dates are formatted correctly in Excel. Use the DATE function if inputting dates manually.
  • Negative results: Swap the order of dates in the formula if your start date is in column B and end date in column A.
  • Unexpected decimals: This is normal and represents partial months. Round the result if you need whole months.
  • If the VBA macro doesn’t run, make sure you’ve enabled macros in Excel’s security settings.

TIP: Remember to save your workbook as an Excel Macro-Enabled Workbook (.xlsm) to retain the VBA code.