“Prepared two hours ago” sounds precise, but it can describe several different intervals. A useful elapsed-time record identifies the preparation event, the endpoint of interest and the clock information used to calculate the difference.
Define what starts the clock
A preparation can involve several recorded events: initial contact with a liquid, completion of a mixing step, transfer into an analytical container and placement into an instrument. These are not necessarily simultaneous.
Choose the event required by the experimental question and name it in the record. If a method defines time zero, preserve that definition rather than substituting whichever timestamp is easiest to retrieve.
University of Iowa Libraries recommends timestamped electronic notebook entries with identifiable authors. That supports attribution of a record, but the time an entry was created is not automatically the time the described preparation occurred.University of Iowa Libraries — Lab Notebooks (opens in a new tab)
For an original example, an entry saved at 10:20 may report preparation completed at 10:05. Both timestamps can be valid if their meanings remain clear; treating the save time as preparation time would shorten the apparent age by 15 minutes.
Calculate the interval for the actual endpoint
| Event | Recorded time | Elapsed from completion |
|---|---|---|
| Preparation completed | 10:05 | 0 minutes |
| Placed in instrument | 10:35 | 30 minutes |
| Measurement started | 11:10 | 65 minutes |
The sample is 30 minutes beyond the chosen starting event when loaded and 65 minutes beyond it when measurement begins. Neither value should be labelled merely “time before analysis” without specifying the endpoint.
For a sequence containing many samples, each measurement can have its own elapsed time. A common loading timestamp does not make the last sample the same age at measurement as the first.
Keep full dates when an interval crosses midnight. Where records come from different clocks or time zones, establish their relationship before subtracting the displayed times.
Preserve incomplete timing honestly
If preparation happened between 10:00 and 10:15 and measurement began at 11:00, the supported interval is 45–60 minutes. Reporting exactly 60 minutes would select one endpoint of an uncertain range without justification.
A remembered estimate can still be recorded, but identify it as an estimate and state its origin. A timestamp copied from a later email is evidence of the email event unless it explicitly documents the earlier activity.
If clock synchronisation is unknown, that limitation belongs alongside the calculation. More decimal places in the result cannot repair uncertainty in the inputs.
Use timing to interpret comparisons
When comparing two runs, check whether their reported sample ages use the same start and endpoint definitions. A 30-minute loading interval and a 30-minute measurement interval describe different histories.
Timing can be an explanatory variable without being the proven cause of an observed difference. Other sample and analytical conditions still need examination.
Keep a calculation trace that names the two source entries and the arithmetic applied. If a timestamp is corrected later, update the derived interval transparently rather than leaving two contradictory versions in circulation.
A defensible elapsed-time statement is short but specific: the named start event, the named endpoint, their recorded times and any uncertainty that materially affects the comparison.
Sources and further detail
- University of Iowa Libraries — Lab Notebooks (opens in a new tab)
Official library guidance checked for timestamped entries and author attribution; the site attributes its checklist to an NCI laboratory-record guide. Event-time calculations are original illustrative examples.
Sources checked 20 September 2026. Worked records are original hypothetical examples, not Novum operating procedures or actual customer events. This article has not undergone independent scientific peer review.