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If a saccade or fixation is only one gaze sample long, the duration will be equal to the interval between two samples (16.666ms on a 60hz tracker). It will start half a sample before the first sample that is included and end half a sample after the last sample.
So in your original question, the first fixation will be two intervals long (not timestamp2-timestamp1, but rather (timestamp2-timestamp1)*2) and the saccade will be one interval long. The saccade will start half way between timestamp2 and timestamp3 and will end between timestamp3 and timestamp4. These are the times used when calculating statistics/metrics (fixation durations, time to first fixation etc.). They are also visible in the Replay module when hovering the mouse over a fixation or in the Visualization module when looking at a gaze plot.
The start and end-times of fixations/saccades are not available in data export.
/Jonas, software architect, Tobii Pro.