Release notes: correct segwit signalling period start conditions

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doc/release-notes.md

@@ -68,10 +68,11 @@ benefits to Bitcoin users.
 
 Activation for the segwit soft fork is being managed using BIP9
 versionbits.  Segwit's version bit is bit 1, and nodes will begin
-tracking which blocks signal support for segwit on 15 November 2016
-(UTC).  If 95% of blocks within a 2,016-block retarget period (about two
-weeks) signal support for segwit, the soft fork will be locked in.
-After another 2,016 blocks, segwit will activate.
+tracking which blocks signal support for segwit at the beginning of the
+first retarget period after segwit's start date of 15 November 2016.  If
+95% of blocks within a 2,016-block retarget period (about two weeks)
+signal support for segwit, the soft fork will be locked in.  After
+another 2,016 blocks, segwit will activate.
 
 For more information about segwit, please see the [segwit FAQ][], the
 [segwit wallet developers guide][] or BIPs [141][BIP141], [143][BIP143],