You don’t say what app version you are using. I am using the latest android beta release (28) and I do not know the definitive way to force the meter to start a new log file except maybe to open the meter case and press the reset button or remove the SD card.
The alternative which may be better/easier for your needs is to keep a handwritten record of the start/stop measurement times for each piece of equipment. This time is displayed by the phone, usually top right. This comes in handy anyway since sometimes the meter will start a new file unexpectedly. The timestamps in log files appear to be accurate. I usually post-process the log files in a spreadsheet type program like Excel anyway, so this isn’t such a bad thing to work out of one file. I create a new column B and since I am in the eastern USA time zone I use the following formula =A9/(60*60*24) – 4/24 +”1/1/1970″ to change the first timestamp starting at A9. Then fill down. The “4” in 4/24 needs to be adjusted to your time zone. I format the new column B to time. I can then split the file into multiple files based on the timestamps. Maybe this helps you.