The start_time field in the URL is a 10 digit number representing the number of seconds since January 1, 1970. This date is known as the epoch. The epoch is a common start of time for UNIX based computer systems. To convert from/to epoch see https://www.epochconverter.com .
Changing the fourth digit by 1 is approximately 11 days.
If you go to far back, you may get a BBO error message.
You can say the URL of a person that you searched for and later change the field in the username= variable. Replace any spaces in the name with "%20", the "%20" is a special HTML code meaning a space.