If you have this data in ServiceNow, check out the new integration we announced at BigFix and Service Now Setup
Otherwise, for this I would recommend the Location Property Wizard. It does a lot more than locations, any key/value pair works well in the wizard. You can paste in a spreadsheet of two columns, where the “key” would be service tag, the “value” would be the contract date, and use a relevance lookup to retrieve the service tag to find the keys.