diff --git a/src/datetime.rs b/src/datetime.rs
index 04279f5..481af5a 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
     }
 
@@ -187,6 +187,27 @@ impl DateTime {
         Ok(Self::from_timestamp(i64::try_from(now.as_secs())?))
     }
 
+    /// Converts this datetime to a different timezone
+    pub fn convert_timezone(&mut self, zone:TimeZone) {
+        let ts = self.timestamp() + zone.as_seconds();
+        *self = Self::from_timestamp(ts);
+        self.zone = zone;
+    }
+
+    /// Converts a Unix timestamp to a `DateTime` struct with the given timezone
+    pub fn from_timestamp_with_zone(ts: i64, zone: TimeZone) -> Self {
+        let mut dt = Self::from_timestamp(ts);
+        dt.convert_timezone(zone);
+        dt
+    }
+
+    /// Creates a `DateTime` from the system time using the given timmezone
+    pub fn now_with_zone(zone: TimeZone) -> Result<Self, Error> {
+        let mut dt = Self::now()?;
+        dt.convert_timezone(zone);
+        Ok(dt)
+    }
+
     #[allow(clippy::missing_panics_doc)]
     /// Gets the day of the week for this `DateTime`
     pub fn weekday(&self) -> Weekday {
@@ -217,6 +238,12 @@ impl DateTime {
             self.year(),
         )
     }
+
+    /// Adds the given number of seconds to this timespec
+    pub fn add_seconds(&mut self, seconds: i64) {
+        let ts = self.timestamp() + seconds;
+        *self = Self::from_timestamp(ts);
+    }
 }
 
 impl fmt::Display for DateTime {
diff --git a/src/zone.rs b/src/zone.rs
index 85ccad1..c33b062 100644
--- a/src/zone.rs
+++ b/src/zone.rs
@@ -91,7 +91,7 @@ impl TimeZone {
             } => {
                 let base = i64::from(*hours) * SECONDS_PER_HOUR + i64::from(*minutes) * 60;
                 if let Sign::Negative = sign {
-                    -base
+                    0 - base
                 } else {
                     base
                 }