diff --git a/src/timesheet/mod.rs b/src/timesheet/mod.rs
index 4f0a9ad..0f76d12 100644
--- a/src/timesheet/mod.rs
+++ b/src/timesheet/mod.rs
@@ -3,9 +3,11 @@ mod configuration;
 mod extract;
 mod point_types;
 mod report;
+mod validation;
 
 pub use config::{Period, RepositoryConfig, TimesheetConfig};
 pub use configuration::{BasicTimesheetConfiguration, TIMESHEET_DIMENSION_NAME, TIMESHEET_TAG};
 pub use extract::extract_timesheets;
 pub use point_types::TimesheetPointType;
 pub use report::{DayReport, DayType, DayWarning, MonthReport, ReportWarning, TimesheetReport};
+pub use validation::find_overlapping_timecards;
diff --git a/src/timesheet/validation.rs b/src/timesheet/validation.rs
new file mode 100644
index 0000000..0eacf06
--- /dev/null
+++ b/src/timesheet/validation.rs
@@ -0,0 +1,111 @@
+use chrono::NaiveTime;
+
+use crate::models::Timecard;
+
+/// Check if two time ranges overlap.
+fn timecards_overlap(a: &Timecard, b: &Timecard) -> bool {
+    a.from_time < b.to_time && b.from_time < a.to_time
+}
+
+/// Find all overlapping timecard pairs for a day.
+/// Returns a list of tuples containing the two overlapping timecards.
+pub fn find_overlapping_timecards(
+    timecards: &[Timecard],
+) -> Vec<((NaiveTime, NaiveTime), (NaiveTime, NaiveTime))> {
+    let mut overlaps = Vec::new();
+    for i in 0..timecards.len() {
+        for j in (i + 1)..timecards.len() {
+            if timecards_overlap(&timecards[i], &timecards[j]) {
+                overlaps.push((
+                    (timecards[i].from_time, timecards[i].to_time),
+                    (timecards[j].from_time, timecards[j].to_time),
+                ));
+            }
+        }
+    }
+    overlaps
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn time(hour: u32, min: u32) -> NaiveTime {
+        NaiveTime::from_hms_opt(hour, min, 0).unwrap()
+    }
+
+    fn card(from_h: u32, from_m: u32, to_h: u32, to_m: u32) -> Timecard {
+        Timecard::new(time(from_h, from_m), time(to_h, to_m))
+    }
+
+    #[test]
+    fn test_no_overlap_adjacent_timecards() {
+        let timecards = vec![card(9, 0, 12, 0), card(13, 0, 17, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        assert!(overlaps.is_empty());
+    }
+
+    #[test]
+    fn test_no_overlap_exact_touch() {
+        // Touching at 12:00 is NOT an overlap (end time = start time)
+        let timecards = vec![card(9, 0, 12, 0), card(12, 0, 17, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        assert!(overlaps.is_empty());
+    }
+
+    #[test]
+    fn test_partial_overlap() {
+        let timecards = vec![card(9, 0, 12, 30), card(12, 0, 13, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        assert_eq!(overlaps.len(), 1);
+        assert_eq!(overlaps[0].0, (time(9, 0), time(12, 30)));
+        assert_eq!(overlaps[0].1, (time(12, 0), time(13, 0)));
+    }
+
+    #[test]
+    fn test_full_containment() {
+        // One timecard fully contains another
+        let timecards = vec![card(9, 0, 17, 0), card(10, 0, 11, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        assert_eq!(overlaps.len(), 1);
+    }
+
+    #[test]
+    fn test_exact_match_overlap() {
+        let timecards = vec![card(9, 0, 12, 0), card(9, 0, 12, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        assert_eq!(overlaps.len(), 1);
+    }
+
+    #[test]
+    fn test_multiple_overlaps_same_day() {
+        // First overlaps with second, and second overlaps with third
+        let timecards = vec![card(9, 0, 11, 0), card(10, 0, 13, 0), card(12, 0, 15, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        // 9-11 overlaps with 10-13, and 10-13 overlaps with 12-15
+        assert_eq!(overlaps.len(), 2);
+    }
+
+    #[test]
+    fn test_single_timecard_no_overlap() {
+        let timecards = vec![card(9, 0, 17, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        assert!(overlaps.is_empty());
+    }
+
+    #[test]
+    fn test_empty_timecards_no_overlap() {
+        let timecards: Vec<Timecard> = vec![];
+        let overlaps = find_overlapping_timecards(&timecards);
+        assert!(overlaps.is_empty());
+    }
+
+    #[test]
+    fn test_three_timecards_all_overlap() {
+        // All three overlap with each other
+        let timecards = vec![card(9, 0, 15, 0), card(10, 0, 16, 0), card(11, 0, 17, 0)];
+        let overlaps = find_overlapping_timecards(&timecards);
+        // 9-15 with 10-16, 9-15 with 11-17, and 10-16 with 11-17
+        assert_eq!(overlaps.len(), 3);
+    }
+}