diff --git a/src/operations/category.service.ts b/src/operations/category.service.ts
index 44f33c5..411d4b7 100644
--- a/src/operations/category.service.ts
+++ b/src/operations/category.service.ts
@@ -1,5 +1,7 @@
 // vim: set tw=80 ts=2 sw=2 sts=2:
 
+import * as moment from 'moment';
+
 import { Injectable } from '@angular/core';
 import { Observable } from 'rxjs/Rx';
 
@@ -13,14 +15,23 @@ export class CategoryService {
     private restangular: Restangular
   ) {}
 
+  formatDate(date: Date|string) {
+    if(date instanceof Date) {
+      return moment(date).format('YYYY-MM-DD');
+    }
+
+    return date;
+  }
+
   query(id: number, minDate: Date = null, maxDate: Date = null): Observable<Category[]> {
-    var dateRange: any;
+    var dateRange: any = {};
+
     if(minDate) {
-      dateRange.begin = minDate;
+      dateRange.begin = this.formatDate(minDate);
     }
 
     if(maxDate) {
-      dateRange.end = maxDate;
+      dateRange.end = this.formatDate(maxDate);
     }
 
     return this.restangular.one('account', id).getList('category', dateRange);