I have a data set that looks like the following that I'd like to expand to a monthly panel data set.

```
ID | start_date | end_date | event_type |
 1 |   01/01/97 | 08/01/98 |          1 |
 2 |   02/01/97 | 10/01/97 |          1 |
 3 |   01/01/96 | 12/01/04 |          2 |

```

Some cases last longer than others. I've figured out how to expand the data to a yearly configuration by pulling out the year from each date and then using:

```
year <- ddply(df, c("ID"), summarize, year = seq(startyear, endyear))

```

followed by:

```
month <- ddply(year, c("ID"), summarize, month = seq(1, 12))

```

The problem with this approach is that it doesn't assign the correct number for the month, i.e. January = 1, and so it doesn't play well with an event data set that I would like to eventually merge it with, where I would be matching on `year`, `ID`, and `month`. Help would be appreciated. Here is a direct link to the data set I am trying to expand (.xls): <http://db.tt/KeLRCzr9>. Hopefully I've included enough information, but please let me know if there is any other information needed.