diff --git a/backend/src/tasks/lights.rs b/backend/src/tasks/lights.rs
index 0c2a4ab..78affce 100644
--- a/backend/src/tasks/lights.rs
+++ b/backend/src/tasks/lights.rs
@@ -1,6 +1,6 @@
 use std::collections::HashMap;
 
-use chrono::{Datelike, Local, NaiveTime, Weekday};
+use chrono::{DateTime, Datelike, Local, NaiveTime, Weekday};
 use common::{ClientMessage, ServerMessage};
 use lighter_lib::{BulbColor, BulbId};
 use lighter_manager::manager::{BulbCommand, BulbManager, BulbSelector};
@@ -143,25 +143,11 @@ async fn wake_task(
     time: NaiveTime,
 ) {
     let now = Local::now();
-    let day_num = day.num_days_from_monday();
-    let now_day = now.weekday();
-    let now_day_num = now_day.num_days_from_monday();
-
-    let mut alarm = now;
-    if day_num >= now_day_num {
-        // next alarm is this week
-        alarm += chrono::Duration::days((day_num - now_day_num).into());
-        alarm = alarm.date().and_time(time).unwrap();
-    } else {
-        // next alarm is next week
-        alarm += chrono::Duration::weeks(1);
-        alarm -= chrono::Duration::days((now_day_num - day_num).into());
-        alarm = alarm.date().and_time(time).unwrap();
-    }
+    let mut alarm = next_alarm(now, day, time);
 
     loop {
         info!("sleeping until {alarm}");
-        sleep((alarm - Local::now()).to_std().unwrap()).await;
+        sleep((alarm - now).to_std().unwrap()).await;
         alarm += chrono::Duration::weeks(1);
 
         // slowly turn up brightness of bulb
@@ -189,6 +175,21 @@ async fn wake_task(
     }
 }
 
+/// Get the next alarm, from a weekday+time schedule.
+fn next_alarm(now: DateTime<Local>, day: Weekday, time: NaiveTime) -> DateTime<Local> {
+    let day_of_alarm = day.num_days_from_monday() as i64;
+    let day_now = now.weekday().num_days_from_monday() as i64;
+
+    let alarm = now + chrono::Duration::days(day_of_alarm - day_now);
+    let mut alarm = alarm.date().and_time(time).unwrap();
+
+    if alarm <= now {
+        alarm += chrono::Duration::weeks(1);
+    }
+
+    alarm
+}
+
 /// Wait until we receive a client request that mutates the given bulb
 async fn wait_for_bulb_command(
     bulb_id: &BulbId,
@@ -210,3 +211,31 @@ async fn wait_for_bulb_command(
         }
     }
 }
+
+#[cfg(test)]
+mod test {
+    use chrono::{offset::TimeZone, Local, NaiveTime, Weekday};
+
+    use super::next_alarm;
+
+    #[test]
+    fn test_alarm_date() {
+        const FMT: &str = "%Y-%m-%d %H:%M";
+        let now = Local.datetime_from_str("2022-10-18 15:30", FMT).unwrap();
+        let test_values = [
+            (Weekday::Tue, (16, 30), "2022-10-18 16:30"),
+            (Weekday::Tue, (14, 30), "2022-10-25 14:30"),
+            (Weekday::Wed, (15, 30), "2022-10-19 15:30"),
+            (Weekday::Mon, (15, 30), "2022-10-24 15:30"),
+        ];
+
+        for (day, (hour, min), expected) in test_values {
+            let expected = Local.datetime_from_str(expected, FMT).unwrap();
+
+            assert_eq!(
+                next_alarm(now, day, NaiveTime::from_hms(hour, min, 0)),
+                expected
+            );
+        }
+    }
+}