Excel auto-truncating timestamps sub milliseconds

You're right that Excel truncates sub-millisecond data (microseconds and nanoseconds) because its datetime format only supports precision down to milliseconds .You’re running into a precision limitation in Excel, not just a formatting issue. The stair-step graphs are the telltale sign of lost precision.

Practical Workaround

Instead of double-clicking the CSV (which forces Excel's auto-formatting), set up a proper import that preserves your full timestamps with Power Query Import.

1. Open a blank Excel workbook (don't double-click the CSV)
2. Go to the Data tab → Get Data → From File → From Text/CSV 
3. Select your CSV file and click Import
4. CRITICAL STEP: In the preview window, find your timestamp column
5. Click the data type icon (usually says "ABC" or "Date") next to the column header
6. Change it from whatever it auto-detected to "Text" 
7. Click Load

This brings all your data in exactly as written in the CSV—no truncation, no auto-formatting. Your timestamps like 2025-12-18 00:08:46.730845 will remain intact as text strings.

Considerations…

If you later convert the text to a datetime format in Excel, it may still truncate sub-milliseconds. Excel's native datetime format only supports up to milliseconds (3 decimal places).
Workaround: Keep timestamps as text for storage, and use Python/pandas for analysis where pd.to_datetime() preserves nanoseconds.

Use Power Query for storage (import as text) and Python for analysis/visualization. This hybrid approach gives you:

• Data Integrity: Timestamps remain intact in Excel.
• Flexibility: Leverage pandas for high-precision calculations/plotting.

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Hope this will help you.