Note: basic time iteration can be done with the "Time Iterating" expressions. You can see these expressions listed under "Time Iteration" in the "Insert Function" dropdown from Controls->Expression in the GUI. This page gives a notion of what the operations do under the covers, in case you need to create a more sophisticated expression.
The "Do It Yourself" Solution
To do this, use the conn_cmfe expression. For example, if you want to make an average over the previous three time slices:
(conn_cmfe(<[-1]id:varname>, meshname) + conn_cmfe(<[-2]id:varname>, meshname) + conn_cmfe(<[-3]id:varname>, meshname)) / 3.
- In this example, conn_cmfe is not specifying the file name, so it will use the currently open file.
- [-1]id -> 'i' means index (as opposed to 'c' for cycle or 't' for time), 'd' means "delta". So altogether it means one time slice previous to the current time slice.