Start forecast demand at first sale

modules/erp/lager/forecast.php

@@ -131,13 +131,15 @@ function lager_forecast_calculate(
     ?DateTimeImmutable $forecastStartDate = null
 ): array
 {
-    $lookbackDays = 365;
+    $firstSaleDate = empty($dailySales) ? null : min(array_keys($dailySales));
+    $lookbackDays = $firstSaleDate === null
+        ? 0
+        : $asOfDate->diff(new DateTimeImmutable($firstSaleDate))->days + 1;
     $halfLifeDays = 60;
     $weightedUnits = 0.0;
     $weightTotal = 0.0;
     $salesUnits = 0.0;
     $salesDays = 0;
-    $firstSaleDate = null;
     $lastSaleDate = null;
 
     for ($age = 0; $age < $lookbackDays; $age++) {
@@ -150,7 +152,6 @@ function lager_forecast_calculate(
         if ($units > 0) {
             $salesUnits += $units;
             $salesDays++;
-            $firstSaleDate = $dateKey;
             $lastSaleDate ??= $dateKey;
         }
     }