From c0ff74a47b5ff943545d4495dd6c85dc3d76a271 Mon Sep 17 00:00:00 2001
From: "Ryan M. Moore" <rmm1047@gmail.com>
Date: Thu, 9 Jan 2025 13:43:12 -0500
Subject: [PATCH] Fix year padding in `to_rfc3339`

See https://github.com/gleam-lang/time/issues/7
---
 src/gleam/time/timestamp.gleam       | 15 ++++++++++-----
 test/gleam/time/timestamp_test.gleam | 24 ++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/src/gleam/time/timestamp.gleam b/src/gleam/time/timestamp.gleam
index a940bac..36fe984 100644
--- a/src/gleam/time/timestamp.gleam
+++ b/src/gleam/time/timestamp.gleam
@@ -170,18 +170,23 @@ pub fn to_rfc3339(timestamp: Timestamp, offset_minutes offset: Int) -> String {
   let offset_minutes = modulo(offset, 60)
   let offset_hours = int.absolute_value(floored_div(offset, 60.0))
 
-  let n = fn(n) { int.to_string(n) |> string.pad_start(2, "0") }
+  let n2 = pad_digit(_, to: 2)
+  let n4 = pad_digit(_, to: 4)
   let out = ""
-  let out = out <> n(years) <> "-" <> n(months) <> "-" <> n(days)
+  let out = out <> n4(years) <> "-" <> n2(months) <> "-" <> n2(days)
   let out = out <> "T"
-  let out = out <> n(hours) <> ":" <> n(minutes) <> ":" <> n(seconds)
+  let out = out <> n2(hours) <> ":" <> n2(minutes) <> ":" <> n2(seconds)
   case int.compare(offset, 0) {
     order.Eq -> out <> "Z"
-    order.Gt -> out <> "+" <> n(offset_hours) <> ":" <> n(offset_minutes)
-    order.Lt -> out <> "-" <> n(offset_hours) <> ":" <> n(offset_minutes)
+    order.Gt -> out <> "+" <> n2(offset_hours) <> ":" <> n2(offset_minutes)
+    order.Lt -> out <> "-" <> n2(offset_hours) <> ":" <> n2(offset_minutes)
   }
 }
 
+fn pad_digit(digit: Int, to desired_length: Int) -> String {
+  int.to_string(digit) |> string.pad_start(desired_length, "0")
+}
+
 fn modulo(n: Int, m: Int) -> Int {
   case int.modulo(n, m) {
     Ok(n) -> n
diff --git a/test/gleam/time/timestamp_test.gleam b/test/gleam/time/timestamp_test.gleam
index c49ceb8..5a0836d 100644
--- a/test/gleam/time/timestamp_test.gleam
+++ b/test/gleam/time/timestamp_test.gleam
@@ -207,3 +207,27 @@ pub fn to_rfc3339_8_test() {
   |> timestamp.to_rfc3339(-120)
   |> should.equal("1970-01-01T02:00:00-02:00")
 }
+
+pub fn to_rfc3339_9_test() {
+  timestamp.from_unix_seconds(-62_167_219_200)
+  |> timestamp.to_rfc3339(0)
+  |> should.equal("0000-01-01T00:00:00Z")
+}
+
+pub fn to_rfc3339_10_test() {
+  timestamp.from_unix_seconds(-62_135_596_800)
+  |> timestamp.to_rfc3339(0)
+  |> should.equal("0001-01-01T00:00:00Z")
+}
+
+pub fn to_rfc3339_11_test() {
+  timestamp.from_unix_seconds(-61_851_600_000)
+  |> timestamp.to_rfc3339(0)
+  |> should.equal("0010-01-01T00:00:00Z")
+}
+
+pub fn to_rfc3339_12_test() {
+  timestamp.from_unix_seconds(-59_011_459_200)
+  |> timestamp.to_rfc3339(0)
+  |> should.equal("0100-01-01T00:00:00Z")
+}