Use the <$action-setfield> widget, like this:
<$button>
<$action-setfield timestamp=<<now "YY.0MM.0DD.0hh0mm0ss">>/>
click me!
</$button>
"timestamp" is the target fieldname in the current tiddler and "YY.0MM.0DD.0hh0mm0ss" is the date format to save. You can change these to suit your specific purposes. If you want to write a field in a tiddler other than the current tiddler, add $tiddler="NameOfTiddler" as another parameter of the <$action-setfield> widget.
enjoy,
-e
Eric Shulman
TiddlyTools / ELS Design Studios
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