diff --git a/src/add.ts b/src/add.ts
index c128a4f..50900be 100644
--- a/src/add.ts
+++ b/src/add.ts
@@ -1,6 +1,6 @@
-import { curry } from './curry'
-import { parseNumber, parseDate } from './parse'
-import { UnitOfTime, unitsOfTime, millisecondsPer } from './unit-of-time'
+import { curry } from './curry.js'
+import { parseNumber, parseDate } from './parse.js'
+import { UnitOfTime, unitsOfTime, millisecondsPer } from './unit-of-time.js'
 
 /**
  * Returns `NaN` when `date` is in the last-representable month,
diff --git a/src/date-descriptor.ts b/src/date-descriptor.ts
index a7e094b..525b22b 100644
--- a/src/date-descriptor.ts
+++ b/src/date-descriptor.ts
@@ -1,4 +1,4 @@
-import { has } from './parse'
+import { has } from './parse.js'
 
 export type DateDescriptor =
   | { year: number }
diff --git a/src/distance.ts b/src/distance.ts
index a2f008b..efb2023 100644
--- a/src/distance.ts
+++ b/src/distance.ts
@@ -1,7 +1,7 @@
-import { get } from './get'
-import { curry } from './curry'
-import { parseDate } from './parse'
-import { UnitOfTime, unitsOfTime, millisecondsPer } from './unit-of-time'
+import { get } from './get.js'
+import { curry } from './curry.js'
+import { parseDate } from './parse.js'
+import { UnitOfTime, unitsOfTime, millisecondsPer } from './unit-of-time.js'
 
 export const distance = curry(function distance(
   unit: UnitOfTime,
diff --git a/src/get.ts b/src/get.ts
index 1b072ac..fa35172 100644
--- a/src/get.ts
+++ b/src/get.ts
@@ -1,6 +1,6 @@
-import { curry } from './curry'
-import { parseDate } from './parse'
-import { AccessibleUnitOfTime, accessibleUnitsOfTime } from './unit-of-time'
+import { curry } from './curry.js'
+import { parseDate } from './parse.js'
+import { AccessibleUnitOfTime, accessibleUnitsOfTime } from './unit-of-time.js'
 
 export const get = curry(function get(
   unit: AccessibleUnitOfTime,
diff --git a/src/index.ts b/src/index.ts
index 1ec7bf6..812b1dd 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -3,12 +3,12 @@
  * Oh dear, another date library
  */
 
-import { of } from './of'
-import { get } from './get'
-import { add } from './add'
-import { subtract } from './subtract'
-import { startOf } from './start-of'
-import { distance } from './distance'
+import { of } from './of.js'
+import { get } from './get.js'
+import { add } from './add.js'
+import { subtract } from './subtract.js'
+import { startOf } from './start-of.js'
+import { distance } from './distance.js'
 
 type D = {
   of: typeof of
diff --git a/src/of.ts b/src/of.ts
index 4cd66f8..2ddb5bb 100644
--- a/src/of.ts
+++ b/src/of.ts
@@ -3,7 +3,7 @@ import {
   dateStringAsDate,
   DateDescriptor,
   isDateDescriptor,
-} from './date-descriptor'
+} from './date-descriptor.js'
 
 export function isDatestringInFormatISO(datestring: string): boolean {
   return /^[-+]?(?:\d{2})?\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}.\d{3}Z$/.test(datestring)
diff --git a/src/start-of.ts b/src/start-of.ts
index 29b4502..94fe3e3 100644
--- a/src/start-of.ts
+++ b/src/start-of.ts
@@ -1,6 +1,6 @@
-import { curry } from './curry'
-import { parseDate } from './parse'
-import { ResetableUnitOfTime, resetableUnitsOfTime } from './unit-of-time'
+import { curry } from './curry.js'
+import { parseDate } from './parse.js'
+import { ResetableUnitOfTime, resetableUnitsOfTime } from './unit-of-time.js'
 
 export const startOf = curry(function startOf(
   unit: ResetableUnitOfTime,
diff --git a/src/subtract.ts b/src/subtract.ts
index 8e07ee4..4d2f2f4 100644
--- a/src/subtract.ts
+++ b/src/subtract.ts
@@ -1,6 +1,6 @@
-import { add } from './add'
-import { curry } from './curry'
-import { UnitOfTime } from './unit-of-time'
+import { add } from './add.js'
+import { curry } from './curry.js'
+import { UnitOfTime } from './unit-of-time.js'
 
 export const subtract = curry(function subtract(
   unit: UnitOfTime,