The arduino has no time information. First I wanted to use NTP, but one HTTP client (the one sending the sensor data to the server) is enough difficulty for this problem. A friend of mine build a board similar to this setup: http://www.ladyada.net/learn/breakoutplus/ds1307rtc.html .
For initializing and reading the date/time information I use the code from http://www.glacialwanderer.com/hobbyrobotics/?p=12 .
Now I have date/time information for writing the sensor data to the SD card in the future.