DATEDIF and DAYS

Why this lesson matters: DATEDIF and DAYS

Measure elapsed time between milestones in appropriate units. In this module, the skill supports a larger goal: Build reliable timelines, ageing reports, deadlines, and working-day calculations. The practical application is to calculate completed months of service.

=DATEDIF(B2,C2,"m")

Build it step by step

Work from the business question toward the formula or tool. Keep the source data separate from the output and make every important assumption visible to the reviewer.

• Confirm that source values are real Excel dates.
• Choose calendar-day or working-day logic deliberately.
• Test month-end, leap-year, and blank-date scenarios.

Check the result and common failure points

Display source dates with an unambiguous format and check serial values when imported dates behave unexpectedly. Also test a known record, a missing or blank value, and a boundary case. Confirm that ranges are the same size, units are consistent, and copied references move only where intended.

• Compare at least one result with a manual calculation or filtered source record.
• Test blanks, zeros, duplicates, errors, and the exact rule boundary.
• Reconcile row counts or totals before relying on the output.

Use the result in a decision

Calculate completed months of service. Label the output clearly, state its period and unit, and explain the action it supports. A technically correct result is only complete when another person can understand, refresh, and review it.