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Upload File: Click "Choose a file" and select the `.xlsx`, `.xls`, or `.csv` file you want to process.
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Specify Header Row: Enter the number of the row that contains your column titles. For example, if your titles are in the first row, enter `1`. The tool will ignore any rows above this.
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Select Date Columns: From the dropdown lists, choose the column that contains the start dates and the column that contains the end dates.
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Choose Result Columns: For each calculation (Duration in Years and Total Months), you can either:
- Select an existing column from the dropdown to overwrite its data.
- Keep `[Create New Column]` selected and type a name for the new column that will be created.
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Process & Download: Click the "Calculate Duration" button. A preview of the first 10 rows will appear. If everything looks correct, click "Download Excel File" to save your processed file.
Privacy Note: All processing is done in your browser. Your data is never uploaded to a server.