diff --git a/CHANGELOG.md b/CHANGELOG.md
index 944ec13..7057668 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,8 @@ The timestamp of `PusTm` is now optional. See Added and Changed section for deta
 - New `UnixTimestamp` abstraction which contains the unix seconds as an `i64`
   and an optional subsecond millisecond counter (`u16`)
 - `MS_PER_DAY` constant.
+- CUC: Added `from_date_time` and `from_unix_stamp` constructors for time provider.
+- CUC: Add `Add<Duration>` and `AddAssign<Duration>` impl for time provider.
 
 ### CDS time module
 
@@ -42,6 +44,9 @@ The timestamp of `PusTm` is now optional. See Added and Changed section for deta
 
 ## Changed
 
+
+- (breaking) `unix_epoch_to_ccsds_epoch`: Expect and return `i64` instead of `u64` now.
+- (breaking) `ccsds_epoch_to_unix_epoch`: Expect and return `i64` instead of `u64` now.
 - (breaking) `PusTmSecondaryHeader`: Timestamp is optional now, which translates to a
   timestamp of size 0.
 - (breaking): `PusTm`: Renamed `time_stamp` method to `timestamp`, also returns
diff --git a/src/time/cuc.rs b/src/time/cuc.rs
index e3aadad..bbacb47 100644
--- a/src/time/cuc.rs
+++ b/src/time/cuc.rs
@@ -3,6 +3,7 @@
 //!
 //! The core data structure to do this is the [TimeProviderCcsdsEpoch] struct.
 use super::*;
+use chrono::Datelike;
 use core::fmt::Debug;
 use core::ops::{Add, AddAssign};
 use core::time::Duration;
@@ -93,6 +94,7 @@ pub fn fractional_part_from_subsec_ns(
 pub enum CucError {
     InvalidCounterWidth(u8),
     InvalidFractionResolution(FractionalResolution),
+    /// Invalid counter supplied.
     InvalidCounter(u8, u64),
     InvalidFractions(FractionalResolution, u64),
 }
@@ -234,7 +236,7 @@ impl TimeProviderCcsdsEpoch {
     #[cfg(feature = "std")]
     pub fn from_now(fraction_resolution: FractionalResolution) -> Result<Self, StdTimestampError> {
         let now = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH)?;
-        let ccsds_epoch = unix_epoch_to_ccsds_epoch(now.as_secs());
+        let ccsds_epoch = unix_epoch_to_ccsds_epoch(now.as_secs() as i64);
         if fraction_resolution == FractionalResolution::Seconds {
             return Ok(Self::new(ccsds_epoch as u32));
         }
@@ -249,7 +251,7 @@ impl TimeProviderCcsdsEpoch {
     #[cfg(feature = "std")]
     pub fn update_from_now(&mut self) -> Result<(), StdTimestampError> {
         let now = SystemTime::now().duration_since(SystemTime::UNIX_EPOCH)?;
-        self.counter.1 = unix_epoch_to_ccsds_epoch(now.as_secs()) as u32;
+        self.counter.1 = unix_epoch_to_ccsds_epoch(now.as_secs() as i64) as u32;
         if self.fractions.is_some() {
             self.fractions = fractional_part_from_subsec_ns(
                 self.fractions.unwrap().0,
@@ -259,6 +261,42 @@ impl TimeProviderCcsdsEpoch {
         Ok(())
     }
 
+    pub fn from_date_time(
+        dt: &DateTime<Utc>,
+        res: FractionalResolution,
+    ) -> Result<Self, TimestampError> {
+        // Year before CCSDS epoch is invalid.
+        if dt.year() < 1958 {
+            return Err(TimestampError::DateBeforeCcsdsEpoch(*dt));
+        }
+        Self::new_generic(
+            WidthCounterPair(4, dt.timestamp() as u32),
+            fractional_part_from_subsec_ns(res, dt.timestamp_subsec_nanos() as u64),
+        )
+        .map_err(|e| e.into())
+    }
+
+    pub fn from_unix_stamp(
+        unix_stamp: &UnixTimestamp,
+        res: FractionalResolution,
+    ) -> Result<Self, TimestampError> {
+        let ccsds_epoch = unix_epoch_to_ccsds_epoch(unix_stamp.unix_seconds);
+        // Negative CCSDS epoch is invalid.
+        if ccsds_epoch < 0 {
+            return Err(TimestampError::DateBeforeCcsdsEpoch(
+                unix_stamp.as_date_time().unwrap(),
+            ));
+        }
+        if ccsds_epoch > u32::MAX as i64 {
+            return Err(CucError::InvalidCounter(4, ccsds_epoch as u64).into());
+        }
+        let mut fractions = None;
+        if let Some(subsec_millis) = unix_stamp.subsecond_millis {
+            fractions = fractional_part_from_subsec_ns(res, subsec_millis as u64 * 10_u64.pow(6));
+        }
+        Self::new_generic(WidthCounterPair(4, ccsds_epoch as u32), fractions).map_err(|e| e.into())
+    }
+
     pub fn new_u16_counter(counter: u16) -> Self {
         // These values are definitely valid, so it is okay to unwrap here.
         Self::new_generic(WidthCounterPair(2, counter as u32), None).unwrap()
@@ -357,7 +395,7 @@ impl TimeProviderCcsdsEpoch {
 
     #[inline]
     fn unix_seconds(&self) -> i64 {
-        ccsds_epoch_to_unix_epoch(self.counter.1 as u64) as i64
+        ccsds_epoch_to_unix_epoch(self.counter.1 as i64)
     }
 
     /// This returns the length of the individual components of the CUC timestamp in addition
@@ -627,6 +665,7 @@ fn get_provider_values_after_duration_addition(
     } else {
         None
     };
+    increment_counter(duration.as_secs() as u32);
     (new_counter, fractional_part)
 }
 
@@ -691,6 +730,7 @@ mod tests {
         let zero_cuc = zero_cuc.unwrap();
         let res = zero_cuc.write_to_bytes(&mut buf);
         assert!(res.is_ok());
+        assert!(zero_cuc.subsecond_millis().is_none());
         assert_eq!(zero_cuc.len_as_bytes(), 5);
         assert_eq!(pfield_len(buf[0]), 1);
         let written = res.unwrap();
@@ -1039,4 +1079,31 @@ mod tests {
         // Assert that the maximum resolution can be reached
         assert_eq!(fractions.1, 2_u32.pow(3 * 8) - 2);
     }
+
+    #[test]
+    fn add_duration_basic() {
+        let mut cuc_stamp = TimeProviderCcsdsEpoch::new(200);
+        cuc_stamp.set_fractional_resolution(FractionalResolution::FifteenUs);
+        let duration = Duration::from_millis(2500);
+        cuc_stamp += duration;
+        assert_eq!(cuc_stamp.width_counter_pair().1, 202);
+        let fractions = cuc_stamp.width_fractions_pair().unwrap().1;
+        let expected_val =
+            (0.5 * fractional_res_to_div(FractionalResolution::FifteenUs) as f64).floor() as u32;
+        assert_eq!(fractions, expected_val);
+        let cuc_stamp2 = cuc_stamp + Duration::from_millis(501);
+        // What I would roughly expect
+        assert_eq!(cuc_stamp2.counter.1, 203);
+        assert!(cuc_stamp2.fractions.unwrap().1 < 100);
+        assert!(cuc_stamp2.subsecond_millis().unwrap() <= 1);
+    }
+
+    #[test]
+    fn add_duration_overflow() {
+        let mut cuc_stamp =
+            TimeProviderCcsdsEpoch::new_generic(WidthCounterPair(1, 255), None).unwrap();
+        let duration = Duration::from_secs(10);
+        cuc_stamp += duration;
+        assert_eq!(cuc_stamp.counter.1, 10);
+    }
 }
diff --git a/src/time/mod.rs b/src/time/mod.rs
index b5dd984..70d7121 100644
--- a/src/time/mod.rs
+++ b/src/time/mod.rs
@@ -170,12 +170,12 @@ pub const fn ccsds_to_unix_days(ccsds_days: i64) -> i64 {
 
 /// Similar to [unix_to_ccsds_days] but converts the epoch instead, which is the number of elpased
 /// seconds since the CCSDS and UNIX epoch times.
-pub const fn unix_epoch_to_ccsds_epoch(unix_epoch: u64) -> u64 {
-    (unix_epoch as i64 - (DAYS_CCSDS_TO_UNIX as i64 * SECONDS_PER_DAY as i64)) as u64
+pub const fn unix_epoch_to_ccsds_epoch(unix_epoch: i64) -> i64 {
+    unix_epoch - (DAYS_CCSDS_TO_UNIX as i64 * SECONDS_PER_DAY as i64)
 }
 
-pub const fn ccsds_epoch_to_unix_epoch(ccsds_epoch: u64) -> u64 {
-    (ccsds_epoch as i64 + (DAYS_CCSDS_TO_UNIX as i64 * SECONDS_PER_DAY as i64)) as u64
+pub const fn ccsds_epoch_to_unix_epoch(ccsds_epoch: i64) -> i64 {
+    ccsds_epoch + (DAYS_CCSDS_TO_UNIX as i64 * SECONDS_PER_DAY as i64)
 }
 
 #[cfg(feature = "std")]
@@ -332,7 +332,7 @@ mod tests {
             .duration_since(SystemTime::UNIX_EPOCH)
             .unwrap();
         let unix_epoch = now.as_secs();
-        let ccsds_epoch = unix_epoch_to_ccsds_epoch(now.as_secs());
+        let ccsds_epoch = unix_epoch_to_ccsds_epoch(now.as_secs() as i64) as u64;
         assert!(ccsds_epoch > unix_epoch);
         assert_eq!((ccsds_epoch - unix_epoch) % SECONDS_PER_DAY as u64, 0);
         let days_diff = (ccsds_epoch - unix_epoch) / SECONDS_PER_DAY as u64;