How the time is set
The time-stamping of data depends on the method of recording. In streaming mode (when you're connected to another device over bluetooth) the time-stamp is derived from the connected device, i.e. and iPhone running the E4 realtime app. In memory recording mode, the timestamp is set by the E4 manager when you connect the E4 via USB. The E4 will be set to the connected device's system clock during pairing. There can be some drift in the clock when the device is powered down so it's best to set it shortly before starting a recording session.
Timestamps in Empatica datafiles (CSV)
The E4 stores time using Unix time format which is a different standard than Excel.
Unix time is a count of seconds from 01 JAN 1970 at 00:00:00 UTC. Excel uses the epoch 01 JAN 1900 and stores date as an integer and time as a decimal.
The formula to convert unix time to Excel friendly is
When the E4 is synchronized with your computer the clock is set to the current time in Coordinated Universal Time (UTC). The E4 does not store the timezone offset so make note of changes in user timezone in your research log.
to adjust the timestamp to local time add ([offset time]/24) to the end of your formula
e.g. for US Eastern Daylight Time:
Adding Timestamps to each value in a time-series
The attached file "EDA+timestamps.xlsx" demonstrates how to add timestamps to each EDA value in an Excel file.
Timestamps in E4 connect
The E4 connect website adjusts timestamps to your browsers' current UTC offset. For example, if you are viewing a file with an actual start time of 13:00 UTC from the US Eastern Standard Time Zone (UTC-5) you will see the file rendered with a start time of 08:00 when you load the session on the E4 connect website.
Synchronization with Video
Empatica E4 includes an Event Marking Button that when triggered caused an the Status LED Light to be Illuminated for 1 second and simultaneously logs a timestamp in the session archive's "tags.csv" file with a 1/64 second resolution. This event mark can be used to align E4 data to video data using the following procedure:
- Press E4 Button on camera
- In Video: ID Timestamp (time v) of LED illumination (first frame visible)
- ID corresponding Event Mark in Empatica log (time e)
- time e - time v = time offset
Image #1 - An E4 Event mark, the red / blue illumination after a button press, as seen in in a video recording.
Image #2 - Event Marks appear as a red veritcal line in the E4 connect Visualization