diff --git a/src/components/calendar.rs b/src/components/calendar.rs
index 4d6b9b32..76d9c18b 100644
--- a/src/components/calendar.rs
+++ b/src/components/calendar.rs
@@ -10,7 +10,7 @@ use crate::{
         duration::{DateDuration, TimeDuration},
         Date, DateTime, Duration, MonthDay, YearMonth,
     },
-    fields::{TemporalFieldKey, TemporalFields},
+    fields::{FieldMap, TemporalFields},
     iso::{IsoDate, IsoDateSlots},
     options::{ArithmeticOverflow, TemporalUnit},
     TemporalError, TemporalResult,
@@ -634,19 +634,16 @@ impl Calendar {
     }
 
     /// Provides field keys to be ignored depending on the calendar.
-    pub fn field_keys_to_ignore(
-        &self,
-        keys: &[TemporalFieldKey],
-    ) -> TemporalResult<Vec<TemporalFieldKey>> {
-        let mut ignored_keys = Vec::with_capacity(5);
+    pub fn field_keys_to_ignore(&self, keys: FieldMap) -> TemporalResult<FieldMap> {
+        let mut ignored_keys = FieldMap::empty();
         if self.is_iso() {
             // NOTE: It is okay for ignored keys to have duplicates?
-            for key in keys {
-                ignored_keys.push(*key);
-                if key == &TemporalFieldKey::Month {
-                    ignored_keys.push(TemporalFieldKey::MonthCode)
-                } else if key == &TemporalFieldKey::MonthCode {
-                    ignored_keys.push(TemporalFieldKey::Month)
+            for key in keys.iter() {
+                ignored_keys.set(key, true);
+                if key == FieldMap::MONTH {
+                    ignored_keys.set(FieldMap::MONTH_CODE, true);
+                } else if key == FieldMap::MONTH_CODE {
+                    ignored_keys.set(FieldMap::MONTH, true);
                 }
             }
 
@@ -698,9 +695,11 @@ impl From<YearMonth> for Calendar {
     }
 }
 
-#[cfg(all(test, feature = "compiled_data"))]
+#[cfg(test)]
 mod tests {
-    use super::*;
+    use crate::{components::Date, iso::IsoDate, options::TemporalUnit};
+
+    use super::Calendar;
 
     #[test]
     fn date_until_largest_year() {
@@ -947,7 +946,7 @@ mod tests {
             ),
         ];
 
-        let calendar = Calendar::from_str("iso8601").unwrap();
+        let calendar = Calendar::default();
 
         for test in tests {
             let first = Date::new_unchecked(
@@ -962,22 +961,22 @@ mod tests {
                 .date_until(&first, &second, TemporalUnit::Year)
                 .unwrap();
             assert_eq!(
-                result.years() as i32,
+                result.years().as_inner() as i32,
                 test.2 .0,
                 "year failed for test \"{test:?}\""
             );
             assert_eq!(
-                result.months() as i32,
+                result.months().as_inner() as i32,
                 test.2 .1,
                 "months failed for test \"{test:?}\""
             );
             assert_eq!(
-                result.weeks() as i32,
+                result.weeks().as_inner() as i32,
                 test.2 .2,
                 "weeks failed for test \"{test:?}\""
             );
             assert_eq!(
-                result.days() as i32,
+                result.days().as_inner() as i32,
                 test.2 .3,
                 "days failed for test \"{test:?}\""
             );
diff --git a/src/components/date.rs b/src/components/date.rs
index 284770ed..296dddd2 100644
--- a/src/components/date.rs
+++ b/src/components/date.rs
@@ -32,6 +32,8 @@ pub struct PartialDate {
     pub(crate) month: Option<i32>,
     pub(crate) month_code: Option<MonthCode>,
     pub(crate) day: Option<i32>,
+    pub(crate) era: Option<TinyAsciiStr<16>>,
+    pub(crate) era_year: Option<i32>,
 }
 
 impl PartialDate {
@@ -41,8 +43,13 @@ impl PartialDate {
         month: Option<i32>,
         month_code: Option<MonthCode>,
         day: Option<i32>,
+        era: Option<TinyAsciiStr<16>>,
+        era_year: Option<i32>,
     ) -> TemporalResult<Self> {
-        if year.is_none() && month.is_none() && month_code.is_none() && day.is_none() {
+        if !(day.is_some()
+            && (month.is_some() || month_code.is_some())
+            && (year.is_some() || (era.is_some() && era_year.is_some())))
+        {
             return Err(TemporalError::r#type()
                 .with_message("A partial date must have at least one defined field."));
         }
@@ -51,6 +58,8 @@ impl PartialDate {
             month,
             month_code,
             day,
+            era,
+            era_year,
         })
     }
 }
diff --git a/src/components/mod.rs b/src/components/mod.rs
index a95f700e..3e43207e 100644
--- a/src/components/mod.rs
+++ b/src/components/mod.rs
@@ -50,6 +50,7 @@ pub use zoneddatetime::ZonedDateTime;
 
 use crate::TemporalError;
 
+// TODO: Update to account for https://tc39.es/proposal-intl-era-monthcode/
 #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
 #[repr(u8)]
 pub enum MonthCode {
diff --git a/src/fields.rs b/src/fields.rs
index c7e255f8..fc3442ed 100644
--- a/src/fields.rs
+++ b/src/fields.rs
@@ -456,16 +456,15 @@ impl TemporalFields {
     /// Merges two `TemporalFields` depending on the calendar.
     #[inline]
     pub fn merge_fields(&self, other: &Self, calendar: &Calendar) -> TemporalResult<Self> {
-        let add_keys = other.keys().collect::<Vec<_>>();
-        let overridden_keys = calendar.field_keys_to_ignore(&add_keys)?;
+        let overridden_keys = calendar.field_keys_to_ignore(other.bit_map)?;
 
         let mut result = Self::default();
 
-        for key in self.keys() {
-            let value = if overridden_keys.contains(&key) {
-                other.get(key)
+        for key in self.bit_map.iter() {
+            let value = if overridden_keys.contains(key) {
+                other.get(key.try_into()?)
             } else {
-                self.get(key)
+                self.get(key.try_into()?)
             };
 
             let Some(value) = value else {
@@ -474,7 +473,7 @@ impl TemporalFields {
                 ));
             };
 
-            result.insert(key, value)?;
+            result.insert(key.try_into()?, value)?;
         }
 
         Ok(result)
@@ -511,6 +510,12 @@ impl From<PartialDate> for TemporalFields {
         if value.day.is_some() {
             bit_map.set(FieldMap::DAY, true)
         };
+        if value.era.is_some() {
+            bit_map.set(FieldMap::ERA, true)
+        }
+        if value.era_year.is_some() {
+            bit_map.set(FieldMap::ERA_YEAR, true)
+        }
 
         Self {
             bit_map,
@@ -518,6 +523,8 @@ impl From<PartialDate> for TemporalFields {
             month: value.month,
             month_code: value.month_code,
             day: value.day,
+            era: value.era,
+            era_year: value.era_year,
             ..Default::default()
         }
     }