A scientific correct structure to reference to a dataset (DS) as an online resource is:
“DS-Creator/s (Publication year YYYY): DS-Title. Online available (data_pre.do?citid=DS-ID) from DFG-FOR816dw. [Date of download: YYYY-MM-DD]”.
Like every reference to an internet resource via URL, you should give the “Date of access” or “Date of download”.
In this way it is easy to trace back whether the dataset had received a new timestamp (“Last updated”) in the meantime as a result of a change, correction or update.
Peters T. and Richter M. (2011): Climate Station Data Pasto Abraham. Available online (data_pre.do?citid=970) from DFG-FOR816dw. [Date of download: 2011-07-23].