diff --git a/src/builtins/core/date.rs b/src/builtins/core/date.rs
index d99ac3046..6b73eb476 100644
--- a/src/builtins/core/date.rs
+++ b/src/builtins/core/date.rs
@@ -279,15 +279,15 @@ impl PlainDate {
             resolved.smallest_unit == Unit::Day && resolved.increment.get() == 1;
         // 12. If roundingGranularityIsNoop is false, then
         if !rounding_granularity_is_noop {
+            let iso_date_time = IsoDateTime::new_unchecked(self.iso, IsoTime::default());
+            let origin_epoch_ns = iso_date_time.as_nanoseconds();
             // a. Let destEpochNs be GetUTCEpochNanoseconds(other.[[ISOYear]], other.[[ISOMonth]], other.[[ISODay]], 0, 0, 0, 0, 0, 0).
             let dest_epoch_ns = other.iso.as_nanoseconds();
             // b. Let dateTime be ISO Date-Time Record { [[Year]]: temporalDate.[[ISOYear]], [[Month]]: temporalDate.[[ISOMonth]], [[Day]]: temporalDate.[[ISODay]], [[Hour]]: 0, [[Minute]]: 0, [[Second]]: 0, [[Millisecond]]: 0, [[Microsecond]]: 0, [[Nanosecond]]: 0 }.
-            let dt = PlainDateTime::new_unchecked(
-                IsoDateTime::new_unchecked(self.iso, IsoTime::default()),
-                self.calendar.clone(),
-            );
+            let dt = PlainDateTime::new_unchecked(iso_date_time, self.calendar.clone());
             // c. Set duration to ? RoundRelativeDuration(duration, destEpochNs, dateTime, calendarRec, unset, settings.[[LargestUnit]], settings.[[RoundingIncrement]], settings.[[SmallestUnit]], settings.[[RoundingMode]]).
             duration = duration.round_relative_duration(
+                origin_epoch_ns,
                 dest_epoch_ns.0,
                 &dt,
                 Option::<(&TimeZone, &NeverProvider)>::None,
diff --git a/src/builtins/core/datetime.rs b/src/builtins/core/datetime.rs
index 819a278a9..a412e11ec 100644
--- a/src/builtins/core/datetime.rs
+++ b/src/builtins/core/datetime.rs
@@ -312,6 +312,7 @@ impl PlainDateTime {
         let dest_epoch_ns = other.iso.as_nanoseconds();
         // 6. Return ? RoundRelativeDuration(diff, destEpochNs, isoDateTime1, unset, calendar, largestUnit, roundingIncrement, smallestUnit, roundingMode).
         diff.round_relative_duration(
+            self.iso.as_nanoseconds(),
             dest_epoch_ns.0,
             self,
             Option::<(&TimeZone, &NeverProvider)>::None,
@@ -336,10 +337,12 @@ impl PlainDateTime {
         if unit == Unit::Nanosecond {
             return FiniteF64::try_from(diff.normalized_time_duration().0);
         }
+        let origin_epoch_ns = self.iso.as_nanoseconds();
         // 5. Let destEpochNs be GetUTCEpochNanoseconds(isoDateTime2).
         let dest_epoch_ns = other.iso.as_nanoseconds();
         // 6. Return ? TotalRelativeDuration(diff, destEpochNs, isoDateTime1, unset, calendar, unit).
         diff.total_relative_duration(
+            origin_epoch_ns,
             dest_epoch_ns.0,
             self,
             Option::<(&TimeZone, &NeverProvider)>::None,
diff --git a/src/builtins/core/duration/normalized.rs b/src/builtins/core/duration/normalized.rs
index 62bcb351a..177a95ee3 100644
--- a/src/builtins/core/duration/normalized.rs
+++ b/src/builtins/core/duration/normalized.rs
@@ -3,6 +3,7 @@
 use core::{cmp, num::NonZeroU128, ops::Add};
 
 use num_traits::AsPrimitive;
+use timezone_provider::epoch_nanoseconds::EpochNanoseconds;
 
 use crate::{
     builtins::core::{timezone::TimeZone, PlainDate, PlainDateTime},
@@ -370,6 +371,7 @@ impl InternalDurationRecord {
     fn nudge_calendar_unit(
         &self,
         sign: Sign,
+        origin_epoch_ns: EpochNanoseconds,
         dest_epoch_ns: i128,
         dt: &PlainDateTime,
         tz: Option<(&TimeZone, &impl TimeZoneProvider)>, // ???
@@ -562,33 +564,43 @@ impl InternalDurationRecord {
             _ => unreachable!(), // TODO: potentially reject with range error?
         };
 
-        // 7. Let start be ? CalendarDateAdd(calendar, isoDateTime.[[ISODate]], startDuration, constrain).
-        let start =
-            dt.calendar()
-                .date_add(&dt.iso.date, &start_duration, ArithmeticOverflow::Constrain)?;
+        let start_epoch_ns = if r1 == 0 {
+            origin_epoch_ns
+        } else {
+            // 7. Let start be ? CalendarDateAdd(calendar, isoDateTime.[[ISODate]], startDuration, constrain).
+            let start = dt.calendar().date_add(
+                &dt.iso.date,
+                &start_duration,
+                ArithmeticOverflow::Constrain,
+            )?;
+            // 9. Let startDateTime be CombineISODateAndTimeRecord(start, isoDateTime.[[Time]]).
+            let start_date_time = IsoDateTime::new_unchecked(start.iso, dt.iso.time);
+            if let Some((tz, provider)) = tz {
+                tz.get_epoch_nanoseconds_for(start_date_time, Disambiguation::Compatible, provider)?
+                    .ns
+            } else {
+                start_date_time.as_nanoseconds()
+            }
+        };
+
         // 8. Let end be ? CalendarDateAdd(calendar, isoDateTime.[[ISODate]], endDuration, constrain).
         let end =
             dt.calendar()
                 .date_add(&dt.iso.date, &end_duration, ArithmeticOverflow::Constrain)?;
-        // 9. Let startDateTime be CombineISODateAndTimeRecord(start, isoDateTime.[[Time]]).
-        let start = IsoDateTime::new_unchecked(start.iso, dt.iso.time);
         // 10. Let endDateTime be CombineISODateAndTimeRecord(end, isoDateTime.[[Time]]).
         let end = IsoDateTime::new_unchecked(end.iso, dt.iso.time);
 
         // 12. Else,
-        let (start_epoch_ns, end_epoch_ns) = if let Some((tz, provider)) = tz {
+        let end_epoch_ns = if let Some((tz, provider)) = tz {
             // a. Let startEpochNs be ? GetEpochNanosecondsFor(timeZone, startDateTime, compatible).
             // b. Let endEpochNs be ? GetEpochNanosecondsFor(timeZone, endDateTime, compatible).
-            let start_epoch_ns =
-                tz.get_epoch_nanoseconds_for(start, Disambiguation::Compatible, provider)?;
-            let end_epoch_ns =
-                tz.get_epoch_nanoseconds_for(end, Disambiguation::Compatible, provider)?;
-            (start_epoch_ns.ns, end_epoch_ns.ns)
+            tz.get_epoch_nanoseconds_for(end, Disambiguation::Compatible, provider)?
+                .ns
         // 11. If timeZoneRec is unset, then
         } else {
             // a. Let startEpochNs be GetUTCEpochNanoseconds(start.[[Year]], start.[[Month]], start.[[Day]], start.[[Hour]], start.[[Minute]], start.[[Second]], start.[[Millisecond]], start.[[Microsecond]], start.[[Nanosecond]]).
             // b. Let endEpochNs be GetUTCEpochNanoseconds(end.[[Year]], end.[[Month]], end.[[Day]], end.[[Hour]], end.[[Minute]], end.[[Second]], end.[[Millisecond]], end.[[Microsecond]], end.[[Nanosecond]]).
-            (start.as_nanoseconds(), end.as_nanoseconds())
+            end.as_nanoseconds()
         };
 
         // TODO: look into handling asserts
@@ -947,6 +959,7 @@ impl InternalDurationRecord {
     #[inline]
     pub(crate) fn round_relative_duration(
         &self,
+        origin_epoch_ns: EpochNanoseconds,
         dest_epoch_ns: i128,
         dt: &PlainDateTime,
         time_zone: Option<(&TimeZone, &impl TimeZoneProvider)>,
@@ -967,7 +980,14 @@ impl InternalDurationRecord {
         let nudge_result = if irregular_length_unit {
             // a. Let record be ? NudgeToCalendarUnit(sign, duration, destEpochNs, isoDateTime, timeZone, calendar, increment, smallestUnit, roundingMode).
             // b. Let nudgeResult be record.[[NudgeResult]].
-            duration.nudge_calendar_unit(sign, dest_epoch_ns, dt, time_zone, options)?
+            duration.nudge_calendar_unit(
+                sign,
+                origin_epoch_ns,
+                dest_epoch_ns,
+                dt,
+                time_zone,
+                options,
+            )?
         } else if let Some((time_zone, time_zone_provider)) = time_zone {
             // 6. Else if timeZone is not unset, then
             //      a. Let nudgeResult be ? NudgeToZonedTime(sign, duration, isoDateTime, timeZone, calendar, increment, smallestUnit, roundingMode).
@@ -1005,6 +1025,7 @@ impl InternalDurationRecord {
     // 7.5.38 TotalRelativeDuration ( duration, destEpochNs, isoDateTime, timeZone, calendar, unit )
     pub(crate) fn total_relative_duration(
         &self,
+        origin_epoch_ns: EpochNanoseconds,
         dest_epoch_ns: i128,
         dt: &PlainDateTime,
         tz: Option<(&TimeZone, &impl TimeZoneProvider)>,
@@ -1017,6 +1038,7 @@ impl InternalDurationRecord {
             // b. Let record be ? NudgeToCalendarUnit(sign, duration, destEpochNs, isoDateTime, timeZone, calendar, 1, unit, trunc).
             let record = self.nudge_calendar_unit(
                 sign,
+                origin_epoch_ns,
                 dest_epoch_ns,
                 dt,
                 tz,
diff --git a/src/builtins/core/year_month.rs b/src/builtins/core/year_month.rs
index af1ac6c7c..2b58c91d4 100644
--- a/src/builtins/core/year_month.rs
+++ b/src/builtins/core/year_month.rs
@@ -330,6 +330,7 @@ impl PlainYearMonth {
             let dest_epoch_ns = target_iso_date_time.as_nanoseconds();
             // d. Set duration to ? RoundRelativeDuration(duration, destEpochNs, isoDateTime, unset, calendar, resolved.[[LargestUnit]], resolved.[[RoundingIncrement]], resolved.[[SmallestUnit]], resolved.[[RoundingMode]]).
             duration = duration.round_relative_duration(
+                iso_date_time.as_nanoseconds(),
                 dest_epoch_ns.as_i128(),
                 &PlainDateTime::new_unchecked(iso_date_time, self.calendar.clone()),
                 Option::<(&TimeZone, &NeverProvider)>::None,
diff --git a/src/builtins/core/zoneddatetime.rs b/src/builtins/core/zoneddatetime.rs
index fa99126c8..f48552f2a 100644
--- a/src/builtins/core/zoneddatetime.rs
+++ b/src/builtins/core/zoneddatetime.rs
@@ -399,6 +399,7 @@ impl ZonedDateTime {
         let iso = self.get_iso_datetime();
         // 5. Return ? RoundRelativeDuration(difference, ns2, dateTime, timeZone, calendar, largestUnit, roundingIncrement, smallestUnit, roundingMode).
         diff.round_relative_duration(
+            *self.epoch_nanoseconds(),
             other.epoch_nanoseconds().as_i128(),
             &PlainDateTime::new_unchecked(iso, self.calendar().clone()),
             Some((self.timezone(), provider)),
@@ -430,6 +431,7 @@ impl ZonedDateTime {
         let iso = self.get_iso_datetime();
         // 4. Return ? TotalRelativeDuration(difference, ns2, dateTime, timeZone, calendar, unit).
         diff.total_relative_duration(
+            *self.epoch_nanoseconds(),
             other.epoch_nanoseconds().as_i128(),
             &PlainDateTime::new_unchecked(iso, self.calendar().clone()),
             Some((self.timezone(), provider)),