Records of Performance (ROP) in dairy production for Ayrshire, Guernsey, Holstein and Jersey breeds were analyzed to estimate constants (age–month subclass averages adjusted for herd differences, time trend and cow culling effect) which can be used for a new Breed Class Average system for simultaneous adjustment of age and month of calving differences. Age, month, and age by month interaction effects were highly significant and thus we conclude that a joint adjustment for age and seasonal variation is necessary. Seasonal variation was sufficiently continuous to warrant age adjustment within individual month. Within breed adjustment for age–month of calving is necessary, but within geographical region and/or lactation number adjustment is not recommended.