During 2009 Oregon Scientific changed their instruction about the data logger capacity the 2010 manual is correct and says;

Frequency in minutes No. of days available for data logging with Memory
available with standard sensors
 1 19
 2  38
 5  97
 10  194
 15  291

Additional sensors will use more of the memory space available and therefore the number of days it can store data. In particular the UV sensor uses a lot of memory and therefore reduces the spave available by as much as half.

If you find that you are having problems with the data logger then we recomend that you follow the following procedure;

First reset the time interval to a new value.
1. Press clock area until DATA LOGGER mode is displayed.
2. Press SET.
3. Press UP / DWN to select frequency of data recording (1 / 2 / 5 / 10 /15).
4. Press SET.
5. The number of days memory will allow for records will be displayed.

Next clear the data logger memeory.
1. Select the clock area of the screen.
2. Press that area till DATA LOGGER is displayed.
3. Press and hold MEM.
4. All LED icons will start to flash, then start to turn off one by one (right to left).

Follow this with a deep reboot - remove the batteries, disconnect the mains adapter and leave for 15 minutes or longer.

Once you have restarted it reset the time interval to your desired new value.