diff --git a/src/eval.rs b/src/eval.rs
index 3186447..78c21a0 100644
--- a/src/eval.rs
+++ b/src/eval.rs
@@ -3,7 +3,7 @@ use std::result;
 
 use chrono::{Datelike, NaiveDate};
 
-use crate::files::commands::{Birthday, Command, DoneDate, Note, Spec, Task};
+use crate::files::commands::{Birthday, Command, DoneDate, Note, Span, Spec, Task, Time};
 use crate::files::{Files, Source, SourcedCommand};
 
 use self::entry::EntryMap;
@@ -16,10 +16,20 @@ mod entry;
 mod formula_spec;
 mod range;
 
-#[derive(Debug, thiserror::Error)]
+#[derive(Debug)]
 pub enum Error {
-    #[error("TODO")]
-    Todo,
+    DeltaInvalidStep {
+        span: Span,
+        start: NaiveDate,
+        start_time: Option<Time>,
+        prev: NaiveDate,
+        prev_time: Option<Time>,
+    },
+    DeltaNoTime {
+        span: Span,
+        start: NaiveDate,
+        prev: NaiveDate,
+    },
 }
 
 type Result<T> = result::Result<T, Error>;
diff --git a/src/eval/delta.rs b/src/eval/delta.rs
index 0080074..e7ab04d 100644
--- a/src/eval/delta.rs
+++ b/src/eval/delta.rs
@@ -1,6 +1,10 @@
 use std::cmp::Ordering;
 
-use crate::files::commands::{self, Spanned, Time, Weekday};
+use chrono::{Datelike, Duration, NaiveDate};
+
+use crate::files::commands::{self, Span, Spanned, Time, Weekday};
+
+use super::{Error, Result};
 
 /// Like [`commands::DeltaStep`] but includes a new constructor,
 /// [`DeltaStep::Time`].
@@ -137,6 +141,122 @@ impl From<&commands::Delta> for Delta {
     }
 }
 
+struct DeltaEval {
+    start: NaiveDate,
+    start_time: Option<Time>,
+    curr: NaiveDate,
+    curr_time: Option<Time>,
+}
+
+impl DeltaEval {
+    fn new(start: NaiveDate, start_time: Option<Time>) -> Self {
+        Self {
+            start,
+            start_time,
+            curr: start,
+            curr_time: start_time,
+        }
+    }
+
+    fn err_step(&self, span: Span) -> Error {
+        Error::DeltaInvalidStep {
+            span,
+            start: self.start,
+            start_time: self.start_time,
+            prev: self.curr,
+            prev_time: self.curr_time,
+        }
+    }
+
+    fn err_time(&self, span: Span) -> Error {
+        Error::DeltaNoTime {
+            span,
+            start: self.start,
+            prev: self.curr,
+        }
+    }
+
+    fn apply(&mut self, step: &Spanned<DeltaStep>) -> Result<()> {
+        match step.value {
+            DeltaStep::Year(n) => self.step_year(step.span, n),
+            DeltaStep::Month(n) => self.step_month(step.span, n),
+            DeltaStep::MonthReverse(n) => self.step_month_reverse(step.span, n),
+            DeltaStep::Day(n) => self.step_day(step.span, n),
+            DeltaStep::Week(n) => self.step_week(step.span, n),
+            DeltaStep::Hour(n) => self.step_hour(step.span, n),
+            DeltaStep::Minute(n) => self.step_minute(step.span, n),
+            DeltaStep::Weekday(n, wd) => self.step_weekday(step.span, n, wd),
+            DeltaStep::Time(time) => self.step_time(step.span, time),
+        }
+    }
+
+    fn step_year(&mut self, span: Span, amount: i32) -> Result<()> {
+        let curr = self.curr;
+        match NaiveDate::from_ymd_opt(curr.year() + amount, curr.month(), curr.day()) {
+            None => Err(self.err_step(span)),
+            Some(next) => {
+                self.curr = next;
+                Ok(())
+            }
+        }
+    }
+
+    fn step_month(&mut self, span: Span, amount: i32) -> Result<()> {
+        let month0 = self.curr.month0() as i32 + amount;
+        let year = self.curr.year() + month0.div_euclid(12);
+        let month0 = month0.rem_euclid(12) as u32;
+        match NaiveDate::from_ymd_opt(year, month0 + 1, self.curr.day()) {
+            None => Err(self.err_step(span)),
+            Some(next) => {
+                self.curr = next;
+                Ok(())
+            }
+        }
+    }
+
+    fn step_month_reverse(&mut self, span: Span, amount: i32) -> Result<()> {
+        todo!()
+    }
+
+    fn step_day(&mut self, span: Span, amount: i32) -> Result<()> {
+        let delta = Duration::days(amount.into());
+        match self.curr.checked_add_signed(delta) {
+            None => Err(self.err_step(span)),
+            Some(next) => {
+                self.curr = next;
+                Ok(())
+            }
+        }
+    }
+
+    fn step_week(&mut self, span: Span, amount: i32) -> Result<()> {
+        let delta = Duration::days((7 * amount).into());
+        match self.curr.checked_add_signed(delta) {
+            None => Err(self.err_step(span)),
+            Some(next) => {
+                self.curr = next;
+                Ok(())
+            }
+        }
+    }
+
+    fn step_hour(&mut self, span: Span, amount: i32) -> Result<()> {
+        todo!()
+    }
+
+    fn step_minute(&mut self, span: Span, amount: i32) -> Result<()> {
+        todo!()
+    }
+
+    fn step_weekday(&mut self, span: Span, amount: i32, weekday: Weekday) -> Result<()> {
+        todo!()
+    }
+
+    fn step_time(&mut self, span: Span, time: Time) -> Result<()> {
+        todo!()
+    }
+}
+
 impl Delta {
     pub fn lower_bound(&self) -> i32 {
         self.steps.iter().map(|step| step.value.lower_bound()).sum()
@@ -145,4 +265,12 @@ impl Delta {
     pub fn upper_bound(&self) -> i32 {
         self.steps.iter().map(|step| step.value.upper_bound()).sum()
     }
+
+    pub fn apply(&self, start: (NaiveDate, Option<Time>)) -> Result<(NaiveDate, Option<Time>)> {
+        let mut eval = DeltaEval::new(start.0, start.1);
+        for step in &self.steps {
+            eval.apply(step)?;
+        }
+        Ok((eval.curr, eval.curr_time))
+    }
 }