So, you’ve imported a load of references from Bookends (or possibly another Ref Mgr app) and all refs has a $PublicationYear. But the latter is a String-type and timeline needs a Date, indeed, the view won’t initialise unless one in-scope note has a $StartDate.
This stamp addresses that setting $StartDate to 1 January 12:00:00 of the $PublicationYear:
$StartDate=date($PublicationYear,1,1,12,0,0);
Of course if you’ve more granular publication dates (e.g. the refs are a monthly journal or such) then adjust the $StartDate accordingly.
Edge case: you don’t want to use $StartDate for the Timeline
Perhaps you need use $StartDate/$EndDate for other purposes, so the values are wrong for the above task. Well, we know Timeline view defaults to using $StartDate/$EndDate. Those attribute choices this can be customised in the view’s settings panel. However, here you hit a chick & egg sequencing problem as you can’t get to that panel until you have a Timeline view tab whilst a Tab will not initialise as Timeline view unless it has at least one valid (default) data value, i.e. a $StartDate in at least one-scope note.
So, you need to set $StartDate in just one note, allowing the view to initialise so you can access and change the settings for your desired Date-type attributes for plotted timeline items. The ‘sacrificial’ $StartDate value you made can now be deleted (or corrected back to its original value) and your Timeline of references is ready to use.