RSU may be delivered in stocks or cash.  You need to check how it was settled either in full or in part.  Basis for RSU settled in stock is established based on FMV captured as compensation on the W-2.  That's when holding period will start.  You should be able to find all the details from the supplemental statements as well as grant/vesting documents provided by the employer.  Everything should reconcile to the comp breakdown for the W-2.