Because I don't have all the parts just yet for my first Arduino project, I started thinking on how thinks should be put together. Then it struck me. Logging data is OK, but if you can't place your data on a time-line (sort of speak) you can't really evaluate.
Now if we are sending the data to a dedicated web-server, that computer will be perfectly capable of tracking date and time. But if we would build a standalone version, with build-in webserver and data logging, we do need the Arduino to keep track of time itself.
It seems like there are several ways to do this: First by letting the Arduino count (but you won't have RTC), Second by synchronizing time over the serial port, third by providing time with a clock (like the DS1307), fourth by getting time over NTP and fifth by getting of a GPS chip.
Now it will really depend on the situation for choosing one of those. When using the Ethernet shield as an internal web-server, it could be a good idea to use NTP (you can even use it to synchronize with an other computer in your own network. But when your building a stand alone version with only SD-card support and no network connection. It could be better to use a clock (like the DS1307). This way you can remove your SD card with your data logged and time stamped, so you can put it in your computer to read out the data when it suits you.
So note to myself, check this links:
Arduino Time library
Network Time Protocol (NTP) Client