Use Bloomberg, available on select computers in the Business Library.

• To find the IPO date of a specific company, type the company's ticker followed by IPO <GO>
• To find IPO and Additional Offering dates for a specific company, type CACT to perform a Corporate Action Search. Change left dropdown from All Securities to Single Security. Enter company's ticker in field to the right. Adjust dates, hit the Enter key, and then Filter Actions to IPO/ADDL. Click on an action to access details about the offering. 
• Use CACT to screen for companies by IPO/Additional Offering date. Type CACT in the command line. Leave the left drop-down reflecting All Securities. Adjust the date. Filter Actions to IPO/ADDL.