"This is the most significant tax filing form introduced since the w-2 70 years ago," CEO Carlos Rodriguez said in this week's earnings webcast. "Twenty-five thousand clients trusted ADP to assist them with ACA."

ADP prepared 60 million W-2 forms during the quarter. But Rodriguez cited a major difference—W-2s draw their information from the human resource system. There were 10 million 1095-Cs prepared by the company for employers and these draw up multiple sources. "This endeavor was no small effort," Rodriguez said.

One way ADP handled its need for resource was to slow down the migration of mid-market clients from their current systems to the cloud-based Workforce Now system.

On the financial side, net income rose to $532.5 million for the most recently ended period, an increase of 8.8 percent from $489.6 million. Quarterly revenue was $3.25 billion, an increase of 7.5 percent from $3.02 billion in last year's corresponding period. Payroll revenue for the first quarter was $2.58 million, up 5 percent from $2.25 billion a year earlier; an increase of 7 percent in constant currencies.