fixes #693: never select a default date before first existing shift in shifts view

includes/pages/user_shifts.php

@@ -47,8 +47,13 @@ function update_ShiftsFilter_timerange(ShiftsFilter $shiftsFilter, $days)
 {
     $start_time = $shiftsFilter->getStartTime();
     if (is_null($start_time)) {
+        $first_day = DateTime::createFromFormat('Y-m-d', $days[0])->getTimestamp();
+        if(time() < $first_day) {
+            $start_time = $first_day;
+        } else {
             $start_time = time();
         }
+    }
 
     $end_time = $shiftsFilter->getEndTime();
     if ($end_time == null) {