From 5eeb3e556ad6a08fc793362c07a707c2382e825d Mon Sep 17 00:00:00 2001
From: Christopher De Vries <devries@idolstarastronomer.com>
Date: Tue, 4 Feb 2025 16:15:04 -0500
Subject: [PATCH 1/3] initial fix for problems I saw, two test failures need to
 examine and add new tests

---
 src/gleam/time/timestamp.gleam | 2 +-
 src/gleam_time_ffi.mjs         | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/gleam/time/timestamp.gleam b/src/gleam/time/timestamp.gleam
index 12ac4ac..271a6fa 100644
--- a/src/gleam/time/timestamp.gleam
+++ b/src/gleam/time/timestamp.gleam
@@ -300,7 +300,7 @@ fn to_calendar_from_offset(
   timestamp: Timestamp,
   offset: Int,
 ) -> #(Int, Int, Int, Int, Int, Int) {
-  let total = timestamp.seconds - { offset * 60 }
+  let total = timestamp.seconds + { offset * 60 }
   let seconds = modulo(total, 60)
   let total_minutes = floored_div(total, 60.0)
   let minutes = modulo(total, 60 * 60) / 60
diff --git a/src/gleam_time_ffi.mjs b/src/gleam_time_ffi.mjs
index 1f09537..27d09aa 100644
--- a/src/gleam_time_ffi.mjs
+++ b/src/gleam_time_ffi.mjs
@@ -7,5 +7,5 @@ export function system_time() {
 }
 
 export function local_time_offset_seconds() {
-  return new Date().getTimezoneOffset() * 60;
+  return new Date().getTimezoneOffset() * -60;
 }

From 791a16dc9f2372b65262b3f76b59e66a8170707a Mon Sep 17 00:00:00 2001
From: Christopher De Vries <devries@idolstarastronomer.com>
Date: Tue, 4 Feb 2025 16:35:42 -0500
Subject: [PATCH 2/3] Fixed the tests, but need to add a couple more tests:

---
 test/gleam/time/timestamp_test.gleam | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/test/gleam/time/timestamp_test.gleam b/test/gleam/time/timestamp_test.gleam
index 95daa6e..09561df 100644
--- a/test/gleam/time/timestamp_test.gleam
+++ b/test/gleam/time/timestamp_test.gleam
@@ -203,15 +203,15 @@ pub fn to_rfc3339_6_test() {
 }
 
 pub fn to_rfc3339_7_test() {
-  timestamp.from_unix_seconds(60 * 60 + 60 * 5)
+  timestamp.from_unix_seconds(-60 * 60 - 60 * 5)
   |> timestamp.to_rfc3339(duration.seconds(65 * 60))
   |> should.equal("1970-01-01T00:00:00+01:05")
 }
 
 pub fn to_rfc3339_8_test() {
   timestamp.from_unix_seconds(0)
-  |> timestamp.to_rfc3339(duration.seconds(-120 * 60))
-  |> should.equal("1970-01-01T02:00:00-02:00")
+  |> timestamp.to_rfc3339(duration.seconds(120 * 60))
+  |> should.equal("1970-01-01T02:00:00+02:00")
 }
 
 pub fn to_rfc3339_9_test() {

From 1600ab15aedacb1e757709917e082d03284f9bad Mon Sep 17 00:00:00 2001
From: Christopher De Vries <devries@idolstarastronomer.com>
Date: Tue, 4 Feb 2025 16:50:55 -0500
Subject: [PATCH 3/3] Added two tests to switch from UTC to EST (-05:00) and
 EST to UTC

---
 test/gleam/time/timestamp_test.gleam | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/test/gleam/time/timestamp_test.gleam b/test/gleam/time/timestamp_test.gleam
index 09561df..e2f3204 100644
--- a/test/gleam/time/timestamp_test.gleam
+++ b/test/gleam/time/timestamp_test.gleam
@@ -356,6 +356,25 @@ pub fn rfc3339_string_timestamp_rfc3339_string_roundtrip_property_test() {
   original_timestamp == roundtrip_timestamp
 }
 
+// Eastern US Timezone round trip tests
+pub fn rfc3339_string_timestamp_rfc3339_string_roundtrip_to_est_test() {
+  let assert Ok(date_time) =
+    timestamp.parse_rfc3339("2025-02-04T13:00:00+00:00")
+
+  date_time
+  |> timestamp.to_rfc3339(duration.seconds(-18_000))
+  |> should.equal("2025-02-04T08:00:00-05:00")
+}
+
+pub fn rfc3339_string_timestamp_rfc3339_string_roundtrip_from_est_test() {
+  let assert Ok(date_time) =
+    timestamp.parse_rfc3339("2025-02-04T13:00:00-05:00")
+
+  date_time
+  |> timestamp.to_rfc3339(calendar.utc_offset)
+  |> should.equal("2025-02-04T18:00:00Z")
+}
+
 // Check against OCaml Ptime reference implementation.
 //
 // These test cases include leap seconds.