DATEDIFF(): How to Calculate the Time Difference Between Two Dates?

Modified on Thu, 23 Apr at 5:00 PM

Follow these steps to compute a date difference (e.g., age, duration, delay):

Step 1: Create the variable

Click “+ New variable”
Enter a name (e.g., Age at inclusion)
Set the type to Numeric
A date difference always returns a number

Step 2: Add a formula

In the “Dynamic variable” panel, click “Add formula”

Step 3: Use the Date difference helper

In the left panel, click “Date difference”
Select the unit you want (day, month, year, etc.)
A formula is automatically inserted:

datediff(start_date, end_date, "unit")

Step 4: Replace the variables

Replace start_date with your earlier date (e.g., Birth_date)
Replace end_date with your later date (e.g., Inclusion_date)

Example: Age at inclusion

datediff(Birth_date, Inclusion_date, "year")

This calculates the patient’s age in years at the time of inclusion.

Step 5: Confirm

Check the “Variable mapping” if needed (usually not needed)
Click “Confirm” to save the formula