I have this overall "process" partially in Workflows and partially in Process Builder.  Workflows have better Time-Dependent workflow action in my experience (willing to be proved wrong on that), couldn't find a comparable way to set a process into motion in Process Builder.  Workflow has a Time-Dependent action that takes place 20 days in advance of the (Targeted) Closed Date.  If this is triggered then it simply checks a box (boolean) on that specific Opportunity record.  The PB starts whenever: 1) a Opp record is changed; 2) that previously mentioned boolean field is marked as "true".  The PB simply posts some 'alerts' to Chatter and then checks another, differnt custom field that was created on that Opp to validate that that process has been ran against that record (to prevent it from triggering more than once).  

  This all worked perfectly for me in the Sandbox, I created Opps that were 24, 23, 22, and 21 days out on a Friday, and came back on a Monday to see the Opps that were set with Closed Dates of 21, 22, and 23 days away all properly triggered their alerts, posted to Chatter, populated the right records with the expected Boolean values.  Everything was good. 

But then I deployed those PBs, Workflows and custom fields to the production environment and have been battling bugs, errors, misfires, permissions issues, ever since.