Hi Gary 

Thanks for posting in the community! Unfortunately what you are trying to do is not currently supported in Lucidchart. However we're always looking for ways to improve Lucidchart and your feedback is the best way for us to do that. Would you mind adding your idea to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration.

The only way to adjust the timeline is to change the markers or start / finish settings (please see below screenshot). Thanks again for posting!