Thanks for your input! Measurement interval is included at the beginning of csv file together with measurement mode used. This was not in prototype video, however, (but is described in manual). Knowing this value time column can be created with a few clicks and the use of Excel formulas. Now to think about it, it would also be possible to create timestamp entries in csv file while they are being prepared for USB storage mode.
Interval is not included with each measurement as this would use up storage and interval is expected to be mostly uniform. Interval value is stored with measurement file. It is in full seconds for long logging periods. For fast measurement rates actual interval between measurements is detected and value has more precision in seconds or milliseconds with decimal digits.
Neither firmware nor software is open source at the moment.