See discussion on rejected 2825833 (を境に).

@@ -31,0 +32 @@
+<s_inf>often ...を境に</s_inf>

daijs (さかい): "２ ものとものとが接する所。また、ある状態と他の状態との分かれ
目。区切り目。境目。「空と海との―」「生死の―をさまよう」→境する"
but there's no equivalent in its きょう entry, except possibly 
"心の状態。境地。"

@@ -29,0 +30 @@
+<stagr>さかい</stagr>
@@ -31 +31,0 @@
-<s_inf>usu. さかい</s_inf>

境 can be a point in time, rather than a physical boundary.
Kojien, Genius and Shogakukan all have this as a separate sense, which they define as 分かれ目.
Examples:
- この戦争を境に、東京と大阪の錦絵新聞はほぼ廃刊。
- 最近、恋人の早苗に対してどことなく冷めている浩太郎は、2012年の文化の日を境に、突然時間を逆走し始める。
Lately, for no particular reason, Kotaro had been losing interest in his girlfriend Sanae. Then, on Culture Day in 2012, time started to run backwards. 
- 妻の死を境にして彼の人生観は大きく変わった。
His view of life changed completely after his wife's death.

@@ -30,0 +31,6 @@
+<s_inf>usu. さかい</s_inf>
+<gloss>turning point</gloss>
+<gloss>watershed</gloss>
+</sense>
+<sense>
+<pos>&n;</pos>