Hi, I think I've figured it out. As said before the data was these two variables including a variable for each called ''inclusiondate'' so it would look like this: ID                              read_date                        inclusion_date 1                               11Jun2009                       09Jan2008 1                               01Jan2010                       09Jan2008 1                               23Feb2010                       09Jan2008 2                               30Nov2011                       08Feb2008 2                               21Dec2009                       08Feb2008 .... I decided at the end to just subtract the two dates from each other and eliminate all the observations that had over 365 days in between. Thanks for your reply but I hope my ''simple'' method would be enough for now. ... View more