DON'T RUN THE FOLLOWING CODE! Unless you're prepared to reboot your TW. It gives you a button to start a timer that every 10 seconds posts the seconds to the subtitle. You might want this feature if you were tracking how much timer you were spending on a project. What's critically missing is a method to turn off the timer! It will keep going even if you change the name of the internal macros, so I guess they must be compiled at the start. I'm thinking a reveal widget that depends on the existence of another tiddler could be used to turn off the timer.
If you hit the button a second time, it will start a second thread (not really a thread), and the subtitle will switch back and forth between the two different cumulative times.
\define timer()
<$action-setfield
$tiddler='$:/SiteSubtitle'
text=<<cum>>
/>
<$set filter="[<cum>add[10]]" select=0 name=cum>
<$action-delay delay='10' actions=<<timer>>>
</action>
</$set>
\end
\define timerstart()
<$action-delay delay='1'>
</$action>
\end
<$vars cum=0>
<$button>
Start timer <<cum>>
<<timer>>
</$button>
</$vars>