You can use this method to insert strings inside a string:
B4X:
Sub Insert(original As String, ins As String, index As Int) As String
Dim pre, suf As String
If index > 0 Then pre = original.SubString2(0, index)
If index < original.Length Then suf = original.SubString(index)
Return pre & ins & suf
End Sub
Your code:
B4X:
Dim str As String = DateUtils.TicksToString(DateTime.Now)
et.Text = Insert(et.Text,str , et.SelectionStart)
et.SelectionStart = et.SelectionStart + str.Length