Just to clarify, the sys_StartTime value represents the time the survey was started. The sys_EndTime value represents the time the survey was ended. The sys_ElapsedTime value represents the total number of seconds the person took to complete the survey.
These variables, created by a survey's server and stored along with a respondent's answers, are special timestamps that represent the number of seconds that have elapsed since January 1, 1970 (midnight UTC/GMT), not counting leap seconds (in ISO 8601: 1970-01-01T00:00:00Z). This timestamp is also known as the Unix epoch, Unix time, POSIX time, or Unix timestamp. We use this number so that if a survey was simultaneously started by individuals in New York City, and Munich, and Lagos, the same number will be stored, regardless of the respondent's time zone.
If you are examining the data in Microsoft Excel®, here is a handy formula to convert this code to a date and time value set to Greenwich Mean Time (GMT/UTC): =(A1 / 86400) + 25569. Don't forget to format the cell to display the date/time in the style you desire.