diff --git a/erpnext/manufacturing/doctype/production_order/production_order.py b/erpnext/manufacturing/doctype/production_order/production_order.py
index 6cd2ad2..529c2a4 100644
--- a/erpnext/manufacturing/doctype/production_order/production_order.py
+++ b/erpnext/manufacturing/doctype/production_order/production_order.py
@@ -176,18 +176,18 @@
 		self.set('operations', [])
 
 		operations = frappe.db.sql("""select operation, description, workstation, idx,
-			hour_rate, time_in_mins, "Pending" as status from `tabBOM Operation` 
+			hour_rate, time_in_mins, "Pending" as status from `tabBOM Operation`
 			where parent = %s order by idx""", self.bom_no, as_dict=1)
 
 		self.set('operations', operations)
 		self.calculate_time()
-		
+
 	def calculate_time(self):
 		bom_qty = frappe.db.get_value("BOM", self.bom_no, "quantity")
-		
+
 		for d in self.get("operations"):
 			d.time_in_mins = flt(d.time_in_mins) / flt(bom_qty) * flt(self.qty)
-			
+
 		self.calculate_operating_cost()
 
 	def get_holidays(self, workstation):
@@ -220,7 +220,9 @@
 			time_log = make_time_log(self.name, d.operation, d.planned_start_time, d.planned_end_time,
 				flt(self.qty) - flt(d.completed_qty), self.project_name, d.workstation, operation_id=d.name)
 
-			self.check_operation_fits_in_working_hours(d)
+			if d.workstation:
+				# validate operating hours if workstation [not mandatory] is specified
+				self.check_operation_fits_in_working_hours(d)
 
 			original_start_time = time_log.from_time
 			while True:
diff --git a/erpnext/manufacturing/doctype/workstation/workstation.py b/erpnext/manufacturing/doctype/workstation/workstation.py
index bc9b190..dae01df 100644
--- a/erpnext/manufacturing/doctype/workstation/workstation.py
+++ b/erpnext/manufacturing/doctype/workstation/workstation.py
@@ -4,7 +4,7 @@
 from __future__ import unicode_literals
 import frappe
 from frappe import _
-from frappe.utils import flt, cint, getdate, formatdate, comma_and, time_diff_in_seconds, get_datetime
+from frappe.utils import flt, cint, getdate, formatdate, comma_and, time_diff_in_seconds, to_timedelta
 from frappe.model.document import Document
 from dateutil.parser import parse
 
@@ -60,7 +60,7 @@
 	workstation = frappe.get_doc("Workstation", workstation)
 
 	for working_hour in workstation.working_hours:
-		slot_length = (get_datetime(working_hour.end_time) - get_datetime(working_hour.start_time)).total_seconds()
+		slot_length = (to_timedelta(working_hour.end_time or "") - to_timedelta(working_hour.start_time or "")).total_seconds()
 		if slot_length >= operation_length:
 			return