diff --git a/src/nexusformat/definitions/base_classes/NXfit_function.nxdl.xml b/src/nexusformat/definitions/base_classes/NXfit_function.nxdl.xml
index 2e333ef..6bbec0e 100644
--- a/src/nexusformat/definitions/base_classes/NXfit_function.nxdl.xml
+++ b/src/nexusformat/definitions/base_classes/NXfit_function.nxdl.xml
@@ -58,30 +58,14 @@ and a BNF of valid grammar.-->
     <group name="fit_parameters" type="NXparameters">
         <field name="PARAMETER" nameType="any" type="NX_CHAR_OR_NUMBER" units="NX_ANY">
             <doc>
-                A parameter for a fit function.
-                This would typically be a variable that
-                is optimized in a fit.
+                A parameter for a fit function. This would typically be
+                a variable that is optimized in a fit.
             </doc>
             <attribute name="description" type="NX_CHAR">
                 <doc>
                     A description of what this parameter represents.
                 </doc>
             </attribute>
-            <attribute name="fixed" type="NX_BOOLEAN">
-                <doc>
-                    If the parameter is held constant, then this attribute should be True.
-                </doc>
-            </attribute>
-            <attribute name="min_value" type="NX_NUMBER">
-                <doc>
-                    The minimal value of the parameter, to be used as a constraint during fitting.
-                </doc>
-            </attribute>
-            <attribute name="max_value" type="NX_NUMBER">
-                <doc>
-                    The maximal value of the parameter, to be used as a constraint during fitting.
-                </doc>
-            </attribute>
         </field>
     </group>
 </definition>
diff --git a/src/nexusformat/definitions/contributed_definitions/NXsensor_scan.nxdl.xml b/src/nexusformat/definitions/contributed_definitions/NXsensor_scan.nxdl.xml
index 0a03843..129c460 100644
--- a/src/nexusformat/definitions/contributed_definitions/NXsensor_scan.nxdl.xml
+++ b/src/nexusformat/definitions/contributed_definitions/NXsensor_scan.nxdl.xml
@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="UTF-8"?>
+<?xml version='1.0' encoding='UTF-8'?>
 <?xml-stylesheet type="text/xsl" href="nxdlformat.xsl"?>
 <!--
 # NeXus - Neutron and X-ray Common Data Format
@@ -72,8 +72,16 @@
             </doc>
         </field>
         <field name="experiment_description" recommended="true"/>
-        <field name="start_time" type="NX_DATE_TIME" units="NX_TIME" recommended="true"/>
-        <field name="end_time" type="NX_DATE_TIME" units="NX_TIME" recommended="true"/>
+        <field name="start_time" type="NX_DATE_TIME" recommended="true">
+            <doc>
+                The start time of the experiment.
+            </doc>
+        </field>
+        <field name="end_time" type="NX_DATE_TIME" recommended="true">
+            <doc>
+                The end time of the experiment.
+            </doc>
+        </field>
         <group type="NXprocess">
             <doc>
                 Define the program that was used to generate the results file(s)
@@ -145,8 +153,8 @@
                 be useful to understand the experiment.
             </doc>
         </group>
-        <group type="NXinstrument">
-            <group type="NXenvironment">
+        <group type="NXinstrument" recommended="true">
+            <group type="NXenvironment" recommended="true">
                 <doc>
                     Describes an environment setup for the experiment.