In Clarity version 16.4.1 the task timeline view options has additional parameters. What do the values chosen in the Options for Timeline Periods (Type) and Start and End Periods show?
These options are used for the per-period metrics calculations on the timeline Work Breakdown Structure (left side) and not the actual timeline.
Per period metrics do not show broken out by period here, only the total for the metric is shown. You can limit the periods and choose Sum of Periods and then the ETC for only those periods only will calculate. If you choose Grand Totals, the total ETC for the entire investment period will calculate.
The actual timeline will show the Investment start and finish dates no matter what dates you enter in the Start Period and End Period. These dates are only for per-period metrics.
The Calendar Periods section is for the Timeline itself. You can select what type of Calendar periods (Weeks, Months..) you want to display in the timeline.
This part has no date parameters and shows the entire investment dates.
Reference: Managing Tasks in Timeline Layout