I am not with SkyWest but the way it works at my carrier is your non-pilot date of hire applies for things like insurance, vacation time and boarding priority. As far as your seniority as a pilot, when you get hired as a pilot you will be on the bottom of the pilot seniority list. This makes sense otherwise you could be a ramper for 2 years and then get hired as a pilot with a seniority that would allow you to hold captain. Other companies may treat date of hire for benefits and boarding priority differently but I can say almost with certainty that your DOH as a pilot is the only one that matters on the seniority list.