. in addition , the company maintains an employee stock purchase plan ( the 201cespp 201d ) pursuant to which eligible employees may purchase shares of the company 2019s common stock on the last day of each bi-annual offering period at a discount of the lower of the closing market value on the first or last day of such offering period . the offering periods run from june 1 through november 30 and from december 1 through may 31 of each year