diff --git a/CHANGELOG.md b/CHANGELOG.md
index b33d6101bc..68cce41c82 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -57,6 +57,9 @@
 
 ### Changed
 
+- @cosmjs/encoding: Change return type of `fromRfc3339` from `ReadonlyDate` to
+  `Date` as the caller becomes the owner of the object and can safely mutate it
+  in any way.
 - @cosmjs/launchpad-ledger: Renamed to @cosmjs/ledger-amino.
 - @cosmjs/ledger-amino: `LedgerSigner.sign` method renamed `signAmino`.
 
diff --git a/packages/encoding/src/rfc3339.ts b/packages/encoding/src/rfc3339.ts
index 1b58fe140e..180ee995e1 100644
--- a/packages/encoding/src/rfc3339.ts
+++ b/packages/encoding/src/rfc3339.ts
@@ -7,7 +7,7 @@ function padded(integer: number, length = 2): string {
   return filled.substring(filled.length - length);
 }
 
-export function fromRfc3339(str: string): ReadonlyDate {
+export function fromRfc3339(str: string): Date {
   const matches = rfc3339Matcher.exec(str);
   if (!matches) {
     throw new Error("Date string is not in RFC3339 format");
@@ -40,9 +40,8 @@ export function fromRfc3339(str: string): ReadonlyDate {
 
   const tzOffset = tzOffsetSign * (tzOffsetHours * 60 + tzOffsetMinutes) * 60; // seconds
 
-  return new ReadonlyDate(
-    ReadonlyDate.UTC(year, month - 1, day, hour, minute, second, milliSeconds) - tzOffset * 1000,
-  );
+  const timestamp = Date.UTC(year, month - 1, day, hour, minute, second, milliSeconds) - tzOffset * 1000;
+  return new Date(timestamp);
 }
 
 export function toRfc3339(date: Date | ReadonlyDate): string {
diff --git a/packages/tendermint-rpc/src/dates.ts b/packages/tendermint-rpc/src/dates.ts
index 24199e4a1c..4a15ccfc51 100644
--- a/packages/tendermint-rpc/src/dates.ts
+++ b/packages/tendermint-rpc/src/dates.ts
@@ -12,8 +12,7 @@ export interface DateWithNanoseconds extends Date {
 }
 
 export function fromRfc3339WithNanoseconds(dateTimeString: string): DateWithNanoseconds {
-  // fromRfc3339 should give the caller a regular Date directly
-  const out: DateWithNanoseconds = new Date(fromRfc3339(dateTimeString).getTime());
+  const out: DateWithNanoseconds = fromRfc3339(dateTimeString);
   const nanosecondsMatch = dateTimeString.match(/\.(\d+)Z$/);
   const nanoseconds = nanosecondsMatch ? nanosecondsMatch[1].slice(3) : "";
   out.nanoseconds = parseInt(nanoseconds.padEnd(6, "0"), 10);