Chapter 2,Verse 28

avyakthadhini bhuthani
vyaktha madhyani bharatha
avyakthanidhananyeva
thtra ka paridevana

Vedanta says that the very words 'birth' and 'death' are illogical.The same goes for 'creation' and 'destruction'.These words must not even exist in our vocabulary.If they do so,it shows our ignorance.As modern science says that "Matter can neither be created nor be destroyed",it can be converted from one form to another.Thus,describing God as a creator itself is a misnomer.
The same logic can be extended to the words "srishti" & "pralayam" God could not and did not create the world.Just like how a carpenter making a chair does not create one per say,but merely converts wood from a tree into one,God did not create the world.To put it in perspective with modern science,even before the big bang,the earth existed.It shall continue to do so forever.Just that it was not,and will not,be in the form that it is in now.A star,after becoming a supernova,shall continue to exist,but in the form of energy.
The world is continually changing.During the course of this transformation,at some particular time,it becomes perceivable,just like water being visible in its solid & liquid states,but not in its gaseous forms.This is considered to be the creation of the world(srishti).Before this,the world was in a form that was not perceivable(avyaktha avastha,meaning intangible state).It is now in a vyaktha avastha,meaning tangible state.At some point in the future,it will again transform into avyaktha avastha and this phenomenon would be considered as "pralayam"(end of the world).
Thus,creation and destruction are nothing but a continuous change between a tangible form and an intangible form.It is not a linear process,but a cyclic process,which would keep repeating over and over again,and hence you cannot find its beginning or its end.
Hence,Lord Krishna tells Arjuna that birth and death of living beings are merely a transformation between a tangible an intangible forms.Beings manifest at birth,and transform to how they were before birth,an unmanifestable form,at their deaths.Thus,he must not grieve over either birth or death.