Assuming your data is located in A2:C20, try the following:

For start of 25% latest times: =SMALL($A$2:$C$20,COUNT($A$2:$C$20)*0.75)

For end of 25% earliest times: =SMALL($A$2:$C$20,COUNT($A$2:$C$20)*0.25)

I think these should do...