diff --git a/cala-cel-interpreter/src/builtins/decimal.rs b/cala-cel-interpreter/src/builtins/decimal.rs
new file mode 100644
index 00000000..1e3cd020
--- /dev/null
+++ b/cala-cel-interpreter/src/builtins/decimal.rs
@@ -0,0 +1,23 @@
+use rust_decimal::Decimal;
+
+use crate::{cel_type::*, error::*, value::*};
+
+use super::assert_arg;
+
+pub fn cast(args: Vec<CelValue>) -> Result<CelValue, CelError> {
+    match args.first() {
+        Some(CelValue::Decimal(d)) => Ok(CelValue::Decimal(*d)),
+        Some(CelValue::String(s)) => Ok(CelValue::Decimal(
+            s.parse()
+                .map_err(|e| CelError::DecimalError(format!("{e:?}")))?,
+        )),
+        Some(v) => Err(CelError::BadType(CelType::Decimal, CelType::from(v))),
+        None => Err(CelError::MissingArgument),
+    }
+}
+
+pub fn add(args: Vec<CelValue>) -> Result<CelValue, CelError> {
+    let a: &Decimal = assert_arg(args.first())?;
+    let b: &Decimal = assert_arg(args.get(1))?;
+    Ok(CelValue::Decimal(a + b))
+}
diff --git a/cala-cel-interpreter/src/builtins.rs b/cala-cel-interpreter/src/builtins/mod.rs
similarity index 50%
rename from cala-cel-interpreter/src/builtins.rs
rename to cala-cel-interpreter/src/builtins/mod.rs
index e5db2fe0..84ea5b77 100644
--- a/cala-cel-interpreter/src/builtins.rs
+++ b/cala-cel-interpreter/src/builtins/mod.rs
@@ -1,8 +1,11 @@
+pub(crate) mod decimal;
+pub(crate) mod timestamp;
+
 use chrono::{NaiveDate, Utc};
 
 use std::sync::Arc;
 
-use super::{cel_type::*, value::*};
+use super::value::*;
 use crate::error::*;
 
 pub(crate) fn date(args: Vec<CelValue>) -> Result<CelValue, CelError> {
@@ -22,30 +25,6 @@ pub(crate) fn uuid(args: Vec<CelValue>) -> Result<CelValue, CelError> {
     ))
 }
 
-pub(crate) mod decimal {
-    use rust_decimal::Decimal;
-
-    use super::*;
-
-    pub fn cast(args: Vec<CelValue>) -> Result<CelValue, CelError> {
-        match args.first() {
-            Some(CelValue::Decimal(d)) => Ok(CelValue::Decimal(*d)),
-            Some(CelValue::String(s)) => Ok(CelValue::Decimal(
-                s.parse()
-                    .map_err(|e| CelError::DecimalError(format!("{e:?}")))?,
-            )),
-            Some(v) => Err(CelError::BadType(CelType::Decimal, CelType::from(v))),
-            None => Err(CelError::MissingArgument),
-        }
-    }
-
-    pub fn add(args: Vec<CelValue>) -> Result<CelValue, CelError> {
-        let a: &Decimal = assert_arg(args.first())?;
-        let b: &Decimal = assert_arg(args.get(1))?;
-        Ok(CelValue::Decimal(a + b))
-    }
-}
-
 fn assert_arg<'a, T: TryFrom<&'a CelValue, Error = CelError>>(
     arg: Option<&'a CelValue>,
 ) -> Result<T, CelError> {
diff --git a/cala-cel-interpreter/src/builtins/timestamp.rs b/cala-cel-interpreter/src/builtins/timestamp.rs
new file mode 100644
index 00000000..d4365ce7
--- /dev/null
+++ b/cala-cel-interpreter/src/builtins/timestamp.rs
@@ -0,0 +1,23 @@
+use crate::{cel_type::*, error::*, value::*};
+
+use super::assert_arg;
+
+pub fn cast(args: Vec<CelValue>) -> Result<CelValue, CelError> {
+    match args.first() {
+        Some(CelValue::String(s)) => Ok(CelValue::Timestamp(
+            s.parse()
+                .map_err(|e| CelError::TimestampError(format!("{e:?}")))?,
+        )),
+        Some(v) => Err(CelError::BadType(CelType::Timestamp, CelType::from(v))),
+        None => Err(CelError::MissingArgument),
+    }
+}
+
+pub fn format(target: &CelValue, args: Vec<CelValue>) -> Result<CelValue, CelError> {
+    if let CelValue::Timestamp(ts) = target {
+        let format: std::sync::Arc<String> = assert_arg(args.first())?;
+        Ok(CelValue::String(ts.format(&format).to_string().into()))
+    } else {
+        Err(CelError::BadType(CelType::Timestamp, CelType::from(target)))
+    }
+}
diff --git a/cala-cel-interpreter/src/cel_type.rs b/cala-cel-interpreter/src/cel_type.rs
index cadf7599..aa3f6a5f 100644
--- a/cala-cel-interpreter/src/cel_type.rs
+++ b/cala-cel-interpreter/src/cel_type.rs
@@ -17,3 +17,23 @@ pub enum CelType {
     Uuid,
     Decimal,
 }
+
+impl CelType {
+    pub(crate) fn package_name(&self) -> &'static str {
+        match self {
+            CelType::Map => "map",
+            CelType::List => "list",
+            CelType::Int => "int",
+            CelType::UInt => "uint",
+            CelType::Double => "double",
+            CelType::String => "string",
+            CelType::Bytes => "bytes",
+            CelType::Bool => "bool",
+            CelType::Null => "null",
+            CelType::Date => "date",
+            CelType::Timestamp => "timestamp",
+            CelType::Uuid => "uuid",
+            CelType::Decimal => "decimal",
+        }
+    }
+}
diff --git a/cala-cel-interpreter/src/context/decimal.rs b/cala-cel-interpreter/src/context/decimal.rs
index d14e9b03..21db864d 100644
--- a/cala-cel-interpreter/src/context/decimal.rs
+++ b/cala-cel-interpreter/src/context/decimal.rs
@@ -7,7 +7,7 @@ use crate::builtins;
 use super::*;
 
 lazy_static! {
-    pub static ref CEL_CONTEXT: CelContext = {
+    pub static ref CEL_PACKAGE: CelPackage = {
         let mut idents = HashMap::new();
         idents.insert(
             SELF_PACKAGE_NAME,
@@ -17,6 +17,7 @@ lazy_static! {
             Cow::Borrowed("Add"),
             ContextItem::Function(Box::new(builtins::decimal::add)),
         );
-        CelContext { idents }
+
+        CelPackage::new(CelContext { idents }, HashMap::new())
     };
 }
diff --git a/cala-cel-interpreter/src/context/mod.rs b/cala-cel-interpreter/src/context/mod.rs
index 219b57c5..72727bbb 100644
--- a/cala-cel-interpreter/src/context/mod.rs
+++ b/cala-cel-interpreter/src/context/mod.rs
@@ -1,12 +1,19 @@
 mod decimal;
+mod package;
+mod timestamp;
 
 use std::{borrow::Cow, collections::HashMap};
 
-use crate::{builtins, error::*, value::*};
+use crate::{builtins, cel_type::CelType, error::*, value::*};
+
+use package::CelPackage;
 
 const SELF_PACKAGE_NAME: Cow<'static, str> = Cow::Borrowed("self");
 
 type CelFunction = Box<dyn Fn(Vec<CelValue>) -> Result<CelValue, CelError> + Sync>;
+pub(crate) type CelMemberFunction =
+    Box<dyn Fn(&CelValue, Vec<CelValue>) -> Result<CelValue, CelError> + Sync>;
+
 #[derive(Debug)]
 pub struct CelContext {
     idents: HashMap<Cow<'static, str>, ContextItem>,
@@ -30,20 +37,37 @@ impl CelContext {
         );
         idents.insert(
             Cow::Borrowed("decimal"),
-            ContextItem::Package(&decimal::CEL_CONTEXT),
+            ContextItem::Package(&decimal::CEL_PACKAGE),
         );
-        Self { idents }
-    }
 
-    pub(crate) fn package_self(&self) -> Result<&ContextItem, CelError> {
-        self.lookup(&SELF_PACKAGE_NAME)
+        idents.insert(
+            Cow::Borrowed("timestamp"),
+            ContextItem::Package(&timestamp::CEL_PACKAGE),
+        );
+
+        Self { idents }
     }
 
-    pub(crate) fn lookup(&self, name: &str) -> Result<&ContextItem, CelError> {
+    pub(crate) fn lookup_ident(&self, name: &str) -> Result<&ContextItem, CelError> {
         self.idents
             .get(name)
             .ok_or_else(|| CelError::UnknownIdent(name.to_string()))
     }
+
+    pub(crate) fn lookup_member_fn(
+        &self,
+        value: &CelValue,
+        name: &str,
+    ) -> Result<&CelMemberFunction, CelError> {
+        let package_name = CelType::from(value).package_name();
+        let package = if let Some(ContextItem::Package(package)) = self.idents.get(package_name) {
+            package
+        } else {
+            return Err(CelError::UnknownPackage(package_name));
+        };
+
+        package.lookup_member(value, name)
+    }
 }
 impl Default for CelContext {
     fn default() -> Self {
@@ -54,7 +78,7 @@ impl Default for CelContext {
 pub(crate) enum ContextItem {
     Value(CelValue),
     Function(CelFunction),
-    Package(&'static CelContext),
+    Package(&'static CelPackage),
 }
 
 impl std::fmt::Debug for ContextItem {
diff --git a/cala-cel-interpreter/src/context/package.rs b/cala-cel-interpreter/src/context/package.rs
new file mode 100644
index 00000000..2d2be639
--- /dev/null
+++ b/cala-cel-interpreter/src/context/package.rs
@@ -0,0 +1,36 @@
+use super::*;
+
+pub struct CelPackage {
+    nested_ctx: CelContext,
+    member_fns: HashMap<&'static str, CelMemberFunction>,
+}
+
+impl CelPackage {
+    pub fn new(
+        nested_ctx: CelContext,
+        member_fns: HashMap<&'static str, CelMemberFunction>,
+    ) -> Self {
+        Self {
+            nested_ctx,
+            member_fns,
+        }
+    }
+
+    pub(crate) fn package_self(&self) -> Result<&ContextItem, CelError> {
+        self.nested_ctx.lookup_ident(&SELF_PACKAGE_NAME)
+    }
+
+    pub(crate) fn lookup(&self, name: &str) -> Result<&ContextItem, CelError> {
+        self.nested_ctx.lookup_ident(name)
+    }
+
+    pub(crate) fn lookup_member(
+        &self,
+        value: &CelValue,
+        name: &str,
+    ) -> Result<&CelMemberFunction, CelError> {
+        self.member_fns
+            .get(name)
+            .ok_or_else(|| CelError::UnknownAttribute(CelType::from(value), name.to_string()))
+    }
+}
diff --git a/cala-cel-interpreter/src/context/timestamp.rs b/cala-cel-interpreter/src/context/timestamp.rs
new file mode 100644
index 00000000..1acb33f9
--- /dev/null
+++ b/cala-cel-interpreter/src/context/timestamp.rs
@@ -0,0 +1,22 @@
+use lazy_static::lazy_static;
+
+use std::collections::HashMap;
+
+use crate::builtins;
+
+use super::*;
+
+lazy_static! {
+    pub static ref CEL_PACKAGE: CelPackage = {
+        let mut idents = HashMap::new();
+        idents.insert(
+            SELF_PACKAGE_NAME,
+            ContextItem::Function(Box::new(builtins::timestamp::cast)),
+        );
+
+        let mut member_fns: HashMap<_, CelMemberFunction> = HashMap::new();
+        member_fns.insert("format", Box::new(builtins::timestamp::format));
+
+        CelPackage::new(CelContext { idents }, member_fns)
+    };
+}
diff --git a/cala-cel-interpreter/src/error.rs b/cala-cel-interpreter/src/error.rs
index 6f2aeef5..556b132b 100644
--- a/cala-cel-interpreter/src/error.rs
+++ b/cala-cel-interpreter/src/error.rs
@@ -21,6 +21,10 @@ pub enum CelError {
     BadType(CelType, CelType),
     #[error("CelError - UnknownIdentifier: {0}")]
     UnknownIdent(String),
+    #[error("CelError - UnknownPackage: No package installed for type '{0}'")]
+    UnknownPackage(&'static str),
+    #[error("CelError - UnknownAttribute: No attribute '{1}' on type {0:?}")]
+    UnknownAttribute(CelType, String),
     #[error("CelError - IllegalTarget")]
     IllegalTarget,
     #[error("CelError - MissingArgument")]
@@ -33,6 +37,8 @@ pub enum CelError {
     UuidError(String),
     #[error("CelError - DecimalError: {0}")]
     DecimalError(String),
+    #[error("CelError - TimestampError: {0}")]
+    TimestampError(String),
     #[error("CelError - NoMatchingOverload: {0}")]
     NoMatchingOverload(String),
     #[error("CelError - Unexpected: {0}")]
diff --git a/cala-cel-interpreter/src/interpreter.rs b/cala-cel-interpreter/src/interpreter.rs
index 5c027469..bb69dcfe 100755
--- a/cala-cel-interpreter/src/interpreter.rs
+++ b/cala-cel-interpreter/src/interpreter.rs
@@ -44,10 +44,10 @@ impl std::fmt::Display for CelExpression {
     }
 }
 
-#[derive(Debug)]
 enum EvalType<'a> {
     Value(CelValue),
     ContextItem(&'a ContextItem),
+    MemberFn(&'a CelValue, &'a CelMemberFunction),
 }
 
 impl<'a> EvalType<'a> {
@@ -124,7 +124,7 @@ fn evaluate_expression_inner<'a>(
             }
             Ok(EvalType::Value(CelValue::from(map)))
         }
-        Ident(name) => Ok(EvalType::ContextItem(ctx.lookup(name)?)),
+        Ident(name) => Ok(EvalType::ContextItem(ctx.lookup_ident(name)?)),
         Literal(val) => Ok(EvalType::Value(CelValue::from(val))),
         Arithmetic(op, left, right) => {
             let left = evaluate_expression(left, ctx)?;
@@ -156,13 +156,18 @@ fn evaluate_member<'a>(
     use ast::Member::*;
     match member {
         Attribute(name) => match target {
-            EvalType::Value(CelValue::Map(map)) => Ok(EvalType::Value(map.get(name))),
+            EvalType::Value(CelValue::Map(map)) if map.contains_key(name) => {
+                Ok(EvalType::Value(map.get(name)))
+            }
             EvalType::ContextItem(ContextItem::Value(CelValue::Map(map))) => {
                 Ok(EvalType::Value(map.get(name)))
             }
             EvalType::ContextItem(ContextItem::Package(p)) => {
                 Ok(EvalType::ContextItem(p.lookup(name)?))
             }
+            EvalType::ContextItem(ContextItem::Value(v)) => {
+                Ok(EvalType::MemberFn(v, ctx.lookup_member_fn(v, name)?))
+            }
             _ => Err(CelError::IllegalTarget),
         },
         FunctionCall(exprs) => match target {
@@ -176,6 +181,13 @@ fn evaluate_member<'a>(
             EvalType::ContextItem(ContextItem::Package(p)) => {
                 evaluate_member(EvalType::ContextItem(p.package_self()?), member, ctx)
             }
+            EvalType::MemberFn(v, f) => {
+                let mut args = Vec::new();
+                for e in exprs {
+                    args.push(evaluate_expression(e, ctx)?.try_value()?)
+                }
+                Ok(EvalType::Value(f(v, args)?))
+            }
             _ => Err(CelError::IllegalTarget),
         },
         _ => unimplemented!(),
@@ -419,4 +431,18 @@ mod tests {
         assert_eq!(expression.evaluate(&context)?, CelValue::Decimal(3.into()));
         Ok(())
     }
+
+    #[test]
+    fn function_on_timestamp() -> anyhow::Result<()> {
+        use chrono::{DateTime, Utc};
+
+        let time: DateTime<Utc> = "1940-12-21T00:00:00Z".parse().unwrap();
+        let mut context = CelContext::new();
+        context.add_variable("now", time);
+
+        let expression = "now.format('%d/%m/%Y')".parse::<CelExpression>().unwrap();
+        assert_eq!(expression.evaluate(&context)?, CelValue::from("21/12/1940"));
+
+        Ok(())
+    }
 }
diff --git a/cala-cel-interpreter/src/value.rs b/cala-cel-interpreter/src/value.rs
index 7ad65222..724114b9 100644
--- a/cala-cel-interpreter/src/value.rs
+++ b/cala-cel-interpreter/src/value.rs
@@ -47,27 +47,6 @@ pub struct CelMap {
     inner: HashMap<CelKey, CelValue>,
 }
 
-#[derive(Debug, PartialEq)]
-pub struct CelArray {
-    inner: Vec<CelValue>,
-}
-
-impl CelArray {
-    pub fn new() -> Self {
-        Self { inner: Vec::new() }
-    }
-
-    pub fn push(&mut self, elem: impl Into<CelValue>) {
-        self.inner.push(elem.into());
-    }
-}
-
-impl Default for CelArray {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
 impl CelMap {
     pub fn new() -> Self {
         Self {
@@ -85,6 +64,10 @@ impl CelMap {
             .cloned()
             .unwrap_or(CelValue::Null)
     }
+
+    pub fn contains_key(&self, key: impl Into<CelKey>) -> bool {
+        self.inner.contains_key(&key.into())
+    }
 }
 
 impl Default for CelMap {
@@ -109,6 +92,27 @@ impl From<CelMap> for CelValue {
     }
 }
 
+#[derive(Debug, PartialEq)]
+pub struct CelArray {
+    inner: Vec<CelValue>,
+}
+
+impl CelArray {
+    pub fn new() -> Self {
+        Self { inner: Vec::new() }
+    }
+
+    pub fn push(&mut self, elem: impl Into<CelValue>) {
+        self.inner.push(elem.into());
+    }
+}
+
+impl Default for CelArray {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
 impl From<i64> for CelValue {
     fn from(i: i64) -> Self {
         CelValue::Int(i)