The only way log would be resetting wold be deleting the log explicitly. Even resetting the device wouldn’t automatically delete the log. I think the creation time doesn’t really matter because unlike a Linux system, OS is an embedded server and I think the SD card library doesn’t use the current time stamp when writing a file. However, the file name indicates the day the file was written. It’s the epoch time divided by 86400 — for example, today (June 11, 2016)’s time stamp is 16963, and the log file generated today would be under name 16963.
When you check log, it defaults to show only the past few days of log records. If you want to change to a longer period you need to set the time period.