diff --git a/js/src/builtin/temporal/PlainYearMonth.cpp b/js/src/builtin/temporal/PlainYearMonth.cpp
index 693ce8e1b98c..be073b3c1027 100644
--- a/js/src/builtin/temporal/PlainYearMonth.cpp
+++ b/js/src/builtin/temporal/PlainYearMonth.cpp
@@ -3578,6 +3578,19 @@ args
 }
 else
 {
+/
+/
+TODO
+:
+Avoid
+creating
+an
+options
+object
+if
+not
+necessary
+.
 options
 =
 NewPlainObjectWithProto
@@ -3696,6 +3709,121 @@ false
 Step
 8
 .
+Rooted
+<
+PlainObject
+*
+>
+fieldsCopy
+(
+cx
+CopyOptions
+(
+cx
+fields
+)
+)
+;
+if
+(
+!
+fieldsCopy
+)
+{
+return
+false
+;
+}
+/
+/
+Step
+9
+.
+Value
+one
+=
+Int32Value
+(
+1
+)
+;
+auto
+handleOne
+=
+Handle
+<
+Value
+>
+:
+:
+fromMarkedLocation
+(
+&
+one
+)
+;
+if
+(
+!
+DefineDataProperty
+(
+cx
+fields
+cx
+-
+>
+names
+(
+)
+.
+day
+handleOne
+)
+)
+{
+return
+false
+;
+}
+/
+/
+Step
+10
+.
+Rooted
+<
+Wrapped
+<
+PlainDateObject
+*
+>
+>
+intermediateDate
+(
+cx
+CalendarDateFromFields
+(
+cx
+calendar
+fields
+)
+)
+;
+if
+(
+!
+intermediateDate
+)
+{
+return
+false
+;
+}
+/
+/
+Step
+11
+.
 int32_t
 sign
 =
@@ -3719,16 +3847,115 @@ days
 ;
 /
 /
-Steps
-9
+FIXME
+:
+spec
+bug
 -
-10
+calendar
+can
+be
+a
+string
+.
+/
+/
+https
+:
+/
+/
+github
+.
+com
+/
+tc39
+/
+proposal
+-
+temporal
+/
+issues
+/
+2619
+/
+/
+Step
+12
 .
 Rooted
 <
 Value
 >
-day
+dateAdd
+(
+cx
+)
+;
+if
+(
+calendar
+.
+isObject
+(
+)
+)
+{
+Rooted
+<
+JSObject
+*
+>
+calendarObj
+(
+cx
+calendar
+.
+toObject
+(
+)
+)
+;
+if
+(
+!
+GetMethod
+(
+cx
+calendarObj
+cx
+-
+>
+names
+(
+)
+.
+dateAdd
+&
+dateAdd
+)
+)
+{
+return
+false
+;
+}
+}
+/
+/
+Steps
+13
+-
+14
+.
+Rooted
+<
+Wrapped
+<
+PlainDateObject
+*
+>
+>
+date
 (
 cx
 )
@@ -3742,60 +3969,314 @@ sign
 {
 /
 /
+|
+intermediateDate
+|
+is
+initialized
+to
+the
+first
+day
+of
+|
+yearMonth
+|
+'
+s
+/
+/
+month
+.
+Compute
+the
+last
+day
+of
+|
+yearMonth
+|
+'
+s
+month
+by
+first
+adding
+one
+/
+/
+month
+and
+then
+subtracting
+one
+day
+.
+/
+/
+/
+/
+This
+is
+roughly
+equivalent
+to
+these
+calls
+:
+/
+/
+/
+/
+js
+>
+var
+ym
+=
+new
+Temporal
+.
+PlainYearMonth
+(
+2023
+1
+)
+;
+/
+/
+js
+>
+ym
+.
+toPlainDate
+(
+{
+day
+:
+1
+}
+)
+.
+add
+(
+{
+months
+:
+1
+}
+)
+.
+subtract
+(
+{
+days
+:
+1
+}
+)
+.
+day
+/
+/
+31
+/
+/
+/
+/
+For
+many
+calendars
+this
+is
+equivalent
+to
+ym
+.
+daysInMonth
+except
+when
+/
+/
+some
+days
+are
+skipped
+for
+example
+consider
+the
+Julian
+-
+to
+-
+Gregorian
+/
+/
+calendar
+transition
+.
+/
+/
+Step
+13
+.
+a
+.
+Duration
+oneMonthDuration
+=
+{
+0
+1
+}
+;
+/
+/
 Step
-9
+13
+.
+b
 .
 Rooted
 <
-Value
+Wrapped
+<
+PlainDateObject
+*
 >
-yearMonthValue
+>
+nextMonth
 (
 cx
-ObjectValue
+CalendarDateAdd
 (
-*
-yearMonth
+cx
+calendar
+intermediateDate
+oneMonthDuration
+dateAdd
 )
 )
 ;
 if
 (
 !
-CalendarDaysInMonth
+nextMonth
+)
+{
+return
+false
+;
+}
+/
+/
+Step
+13
+.
+c
+.
+Duration
+minusDayDuration
+=
+{
+0
+0
+0
+-
+1
+}
+;
+/
+/
+Step
+13
+.
+d
+.
+Rooted
+<
+Wrapped
+<
+PlainDateObject
+*
+>
+>
+endOfMonth
+(
+cx
+CalendarDateAdd
 (
 cx
 calendar
-yearMonthValue
-&
-day
+nextMonth
+minusDayDuration
+dateAdd
 )
 )
+;
+if
+(
+!
+endOfMonth
+)
 {
 return
 false
 ;
 }
-}
-else
-{
 /
 /
 Step
-10
+13
 .
-day
+e
 .
-setInt32
+Rooted
+<
+Value
+>
+endOfMonthValue
 (
-1
+cx
+ObjectValue
+(
+*
+endOfMonth
 )
+)
+;
+Rooted
+<
+Value
+>
+day
+(
+cx
+)
+;
+if
+(
+!
+CalendarDay
+(
+cx
+calendar
+endOfMonthValue
+&
+day
+)
+)
+{
+return
+false
 ;
 }
 /
 /
 Step
-11
+13
+.
+f
 .
 if
 (
@@ -3803,7 +4284,7 @@ if
 DefineDataProperty
 (
 cx
-fields
+fieldsCopy
 cx
 -
 >
@@ -3823,25 +4304,17 @@ false
 /
 /
 Step
-12
+13
+.
+g
 .
-Rooted
-<
-Wrapped
-<
-PlainDateObject
-*
->
->
 date
-(
-cx
+=
 CalendarDateFromFields
 (
 cx
 calendar
-fields
-)
+fieldsCopy
 )
 ;
 if
@@ -3854,10 +4327,25 @@ return
 false
 ;
 }
+}
+else
+{
 /
 /
 Step
-13
+14
+.
+a
+.
+date
+=
+intermediateDate
+;
+}
+/
+/
+Step
+15
 .
 Rooted
 <
@@ -3899,8 +4387,38 @@ false
 }
 /
 /
+FIXME
+:
+spec
+issue
+-
+unnecessary
+call
+to
+GetOptionsObject
+/
+/
+https
+:
+/
+/
+github
+.
+com
+/
+tc39
+/
+proposal
+-
+temporal
+/
+issues
+/
+2620
+/
+/
 Step
-14
+16
 .
 Rooted
 <
@@ -3930,7 +4448,7 @@ false
 /
 /
 Step
-15
+17
 .
 Rooted
 <
@@ -3950,6 +4468,7 @@ calendar
 date
 durationToAdd
 options
+dateAdd
 )
 )
 ;
@@ -3966,7 +4485,7 @@ false
 /
 /
 Step
-16
+18
 .
 Rooted
 <
@@ -3997,7 +4516,7 @@ false
 /
 /
 Step
-17
+19
 .
 auto
 obj