Their data set is downloadable from http://eml.berkeley.edu//~dromer/#data in .xlsx format. They start with a data set that has one observation for every FOMC meeting and so are irregularly spaced. After some initial transformations, they convert this into a monthly data series as follows:
How can I get RATS to read a series of irregularly-spaced data, and then convert it to monthly frequency using the rules that they specify? (Their file contains the FOMC meeting dates. )"We assign each observation to the month in which the corresponding FOMC meeting occurred. If there are two meetings in a month, we sum the observations. If there are no meetings in a month, we record the observation as zero for that month."