Excel table function: if then or

i am crunching a large dataset. one column is "date". i want to have a column of Invoice Date. for any transaction happened in january, i want the invoice date to be Jan-31-2017...so on and so forth.

• Jan 2, 2017 (invoiceDate Jan 31, 2017)
• Jan 18, 2017 (invoiceDate Jan 31, 2017)
• Feb 5, 2017 (invoiceDate Feb 28,2017) ......

How to write the If function? thanks.