In updating your resume you may have skills from your early career but due to you working and think nothing about it at the time may be something unforeseen like being fired or not getting a job or interviewee asking tough questions, you may forget simple things like date when you did the work etc. reference or networks that may have help you secure the job may help in filling in those information as while you may not remember the date if you have a relevant reference to that specific work experience everything that you do not remember about that specific work experience the reference can fill in that information