Just a thought, but perhaps it's the type of content. Your active cell may contain a datestamp, but instr uses a string inside of a string. Perhaps try using this:
Dim Ti As Variant
Dim C As Integer
Ti = ActiveCell.Value()
C = Instr(1, Str(Ti), ":", 1)
MsgBox (C)
Maybe forcing it into a string form will fix your problem. I haven't tested the solution, so let us know if it works.