diff --git a/src/datetime.rs b/src/datetime.rs
index 481af5a..f961112 100644
--- a/src/datetime.rs
+++ b/src/datetime.rs
@@ -89,7 +89,7 @@ impl DateTime {
     /// Gets the Unix timestamp corresponding to this `DateTime`
     pub fn timestamp(&self) -> i64 {
         let mut seconds = self.timestamp_naive();
-        seconds -= self.zone.as_seconds();
+        seconds += self.zone.as_seconds();
         seconds
     }
 
@@ -188,7 +188,7 @@ impl DateTime {
     }
 
     /// Converts this datetime to a different timezone
-    pub fn convert_timezone(&mut self, zone:TimeZone) {
+    pub fn convert_timezone(&mut self, zone: TimeZone) {
         let ts = self.timestamp() + zone.as_seconds();
         *self = Self::from_timestamp(ts);
         self.zone = zone;
@@ -279,6 +279,7 @@ impl PartialOrd for DateTime {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::zone::Sign;
 
     #[test]
     fn weekday() {
@@ -390,7 +391,7 @@ mod tests {
         let mut dt = DateTime::from_timestamp(1706571482);
         assert_eq!(dt.to_string(), "2024-01-29T23:38:02Z");
         dt.zone = TimeZone::Offset {
-            sign: crate::zone::Sign::Positive,
+            sign: Sign::Positive,
             hours: 4,
             minutes: 15,
         };
@@ -402,10 +403,59 @@ mod tests {
         let mut dt = DateTime::from_timestamp(1706571482);
         assert_eq!(dt.display(), "Mon Jan 29 23:38:02 UTC 2024");
         dt.zone = TimeZone::Offset {
-            sign: crate::zone::Sign::Positive,
+            sign: Sign::Positive,
             hours: 4,
             minutes: 15,
         };
         assert_eq!(dt.display(), "Mon Jan 29 23:38:02 +04:15 2024");
     }
+
+    #[test]
+    fn convert_zone() {
+        let mut dt = DateTime {
+            year: Year::new(2024),
+            month: Month::Janurary,
+            day: 29,
+            hour: 23,
+            minute: 38,
+            second: 02,
+            zone: TimeZone::Utc,
+        };
+        let tz = TimeZone::Offset {
+            sign: Sign::Negative,
+            hours: 5,
+            minutes: 0,
+        };
+        dt.convert_timezone(tz);
+        assert_eq!(
+            dt,
+            DateTime {
+                year: Year::new(2024),
+                month: Month::Janurary,
+                day: 29,
+                hour: 18,
+                minute: 38,
+                second: 02,
+                zone: tz,
+            }
+        );
+    }
+
+    #[test]
+    fn from_timestamp_with_zone() {
+        let tz = TimeZone::Offset { sign: Sign::Negative, hours: 5, minutes: 0 };
+        let dt = DateTime::from_timestamp_with_zone(1706571482, tz);
+        assert_eq!(
+            dt,
+            DateTime {
+                year: Year::new(2024),
+                month: Month::Janurary,
+                day: 29,
+                hour: 18,
+                minute: 38,
+                second: 02,
+                zone: tz,
+            }
+        );
+    }
 }
diff --git a/src/zone.rs b/src/zone.rs
index c33b062..a1fe72c 100644
--- a/src/zone.rs
+++ b/src/zone.rs
@@ -168,3 +168,14 @@ impl fmt::Display for TZError {
 }
 
 impl error::Error for TZError {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn as_seconds() {
+        let tz = TimeZone::Offset { sign: Sign::Negative, hours: 5, minutes: 0 };
+        assert_eq!(tz.as_seconds(), -18000);
+    }
+}