diff --git a/src/main.rs b/src/main.rs
index 863e3d7..0b3c0d6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -81,7 +81,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 
 fn aggregate_values(conn: &mut PooledConn, date: NaiveDate) -> Result<(), Box<dyn std::error::Error>>  {
 
-    let start_time = Utc.with_ymd_and_hms(date.year(), date.month(), date.day(), 00, 00, 00).unwrap() - Duration::minutes(15);
+    let average_duration = 15;
+    let start_time = Utc.with_ymd_and_hms(date.year(), date.month(), date.day(), 00, 00, 00).unwrap() - (Duration::minutes(average_duration) / 2);
     let end_time = start_time + Duration::days(1);
 
     let sensors = [
@@ -89,7 +90,6 @@ fn aggregate_values(conn: &mut PooledConn, date: NaiveDate) -> Result<(), Box<dy
         Sensor::new("maison".to_string(), "bureau".to_string(), "tvoc".to_string())
     ];
 
-    let average_duration = 15;
     for sensor in &sensors {
         let mut current_time = start_time;