This tutorial shows how to use the Media Time Offset variable to speed up a 10 second segment in the middle of a 30 second video clip.  Click here to download timemjpg.zsv (7mb) or use your own video clip with audio to follow along.

Step 1
Import timemjpg.avi using either the file tab or the browse dialog (an icon for this is located at the lower left corner of the ZS4 screen next to the help icon).  Drag the green arrow into the track arrangement area.


 
Step 2
If this is to be the only video clip in this project (and in this case it is), you might want to change the project dimensions.  To do this, click the i icon on the MediaMixer track to access the track properties tab and fill new dimensions into the video width and height entry fields.  Then press update to confirm.  I used 240x180 because those are the dimensions of timemjpg.avi.


 
Step 3
a) Select the timemjpg.avi track as the current track.  Do this either by clicking it in the project treeview tab, or hover the mouse over the track in the track arrangement area and press enter.  Note: keyboard shortcuts affect only the item under the mouse.
b) Select the vars tab (next to f/x tab)
c) Use the exact position controls under the track arrangement to go to a position 10 seconds into the clip.  Type 10 in the entry field and the press = button.


 
Step 4
Set a keyframe of 0 for the Media Time Offset variable.  0 is the default value, in this case we're saying that at 10 seconds, the video is still playing normally. 
The key frame with value 0 can most easily be set by hovering the mouse over the Media Time Offset strip on the vars tab and pressing enter on the keyboard.


 
Step 5
a) Change the current position to 15 the same way as last time, using the positioning controls below the track arrangement panel.
b) Click the variable settings icon to the right of the Media Time Offset variable strip.  This brings up the settings tab at the top.


 
Step 6
Create a key frame with a value of 7.  This can be done by typing 7 into the change entry field and pressing the = button.  Since we are at position 15 seconds into the video, adding 7 to the media time offset means that the clip time is actually at 22 (15+7) seconds at this position, which means that the video will play 12 seconds worth of footage (video+audio) in a period of 5 seconds, which means it will have to speed up for this section.  After 15 seconds it will go back to normal speed.


 
Step 7
To make the video speed up and slow down gradually instead of suddenly, an interpolation setting of quadratic will help.  Every variable has several possible interpolation settings, the default is linear.  This can be changed to quadratic by pressing the arrow buttons next to "linear" which appears above the change value area.  This need only be done once per variable and can be done after setting all values or before.


 
Step 8
To export the resulting video as per usual, click the export icon on the MediaMixer track (its the round one) and use the browse button to select a file name and location.  Click export to go to the next page where you can choose a compression option (use IYUV for uncompressed, mjpg for relatively compressed files).


 
Thats it.  If you have any questions please write them below in the comment field.
For more on the Media Time Offset variable see this page.