I would suggest changing this into ternary operator:
<asp:LinkButton ID="taskLinkButton"
Text='<%# (Eval("LastWorked").ToString().Length > 0) ? Eval("LastWorked").ToString().Substring(0, 9) : string.Empty %>'
runat="server" />
The reason why your approach is not working, I guess, is the fact that it is not actually returning anything.