diff --git a/src/operations/category.service.ts b/src/operations/category.service.ts
index c7559df..adcae12 100644
--- a/src/operations/category.service.ts
+++ b/src/operations/category.service.ts
@@ -23,7 +23,7 @@ export class CategoryService {
     return date;
   }
 
-  query(id: number, minDate: Date = null, maxDate: Date = null): Observable<Category[]> {
+  query(id: number, minDate: Date|string = null, maxDate: Date|string = null): Observable<Category[]> {
     let params: HttpParams = new HttpParams();
 
     if(minDate) {