XPS 15 9530 now requires pressing power button multiple times to start

Until recently, my XPS 15 9530 purchased Aug 2015 has always started up from shutdown or sleep/hibernation with a single press of the power button.  Lately however, startup requires a long duration >15 sec press and hold, usually followed by another press before the white LED illuminates.  Once it starts, boot-up and OS all come up with no issues.  Ran a full Dell pre-OS diagnostics and passed.  Upgraded BIOS from A09 to A12.  Power-up issue persists.  Based on Community comments, I was planning to remove cover to measure CMOS back-up battery as a next step.  Any ideas?

UPDATE:  This issue occurs when the computer is left in shutdown or hibernate/sleep for extended period (like overnight).  If I shut it down and immediately restart, the single button push works.

UPDATE:  Computer time and date info is rock solid, which other posters are saying points to failing CMOS back-up.  This issue seemed to be occurring for the better part of last week, but does not seem to be a hard failure -- this morning I did a cold start after shutdown for ~ 8hrs and it worked on the first press :-S.