Data Logging with SD Cards - Exploring Arduino: Tools and Techniques for Engineering Wizardry - page 300

UsingtheReal-TimeClock
Now it is time to combine the SD card and real-time clock, along with the
RTC library that you just downloaded, to enable logging using actual time-
stamps. You update your sketch once again to use the RTC values rather than the
millis values.
Installing the RTC and SD Card Modules
First, ensure that both your SD card shield and RTC are connected to your
Arduino. If you are using the Cooking Hacks SD shield and the adafruit RTC
shield, it should look something like Figure 13-20.
Figure 13-20: Arduino with mounted SD card and RTC breakout boards