diff --git a/reference/datetime/functions/gmmktime.xml b/reference/datetime/functions/gmmktime.xml
index b15e6af373..3697548273 100644
--- a/reference/datetime/functions/gmmktime.xml
+++ b/reference/datetime/functions/gmmktime.xml
@@ -38,7 +38,10 @@
      <term><parameter>hour</parameter></term>
      <listitem>
       <para>
-       The hour
+       The number of the hour relevant to the start of the day determined by
+       <parameter>month</parameter>, <parameter>day</parameter> and <parameter>year</parameter>.
+       Negative values reference the hour before midnight of the day in question.
+       Values greater than 23 reference the appropriate hour in the following day(s).
       </para>
      </listitem>
     </varlistentry>
@@ -46,7 +49,9 @@
      <term><parameter>minute</parameter></term>
      <listitem>
       <para>
-       The minute
+       The number of the minute relevant to the start of the <parameter>hour</parameter>.
+       Negative values reference the minute in the previous hour.
+       Values greater than 59 reference the appropriate minute in the following hour(s).
       </para>
      </listitem>
     </varlistentry>
@@ -54,7 +59,9 @@
      <term><parameter>second</parameter></term>
      <listitem>
       <para>
-       The second
+       The number of seconds relevant to the start of the <parameter>minute</parameter>.
+       Negative values reference the second in the previous minute.
+       Values greater than 59 reference the appropriate second in the following minute(s).
       </para>
      </listitem>
     </varlistentry>
@@ -62,7 +69,10 @@
      <term><parameter>month</parameter></term>
      <listitem>
       <para>
-       The month
+       The number of the month relevant to the end of the previous year.
+       Values 1 to 12 reference the normal calendar months of the year in question.
+       Values less than 1 (including negative values) reference the months in the previous year in reverse order, so 0 is December, -1 is November, etc.
+       Values greater than 12 reference the appropriate month in the following year(s).
       </para>
      </listitem>
     </varlistentry>
@@ -70,7 +80,10 @@
      <term><parameter>day</parameter></term>
      <listitem>
       <para>
-       The day
+       The number of the day relevant to the end of the previous month.
+       Values 1 to 28, 29, 30 or 31 (depending upon the month) reference the normal days in the relevant month.
+       Values less than 1 (including negative values) reference the days in the previous month, so 0 is the last day of the previous month, -1 is the day before that, etc.
+       Values greater than the number of days in the relevant month reference the appropriate day in the following month(s).
       </para>
      </listitem>
     </varlistentry>
diff --git a/reference/datetime/functions/mktime.xml b/reference/datetime/functions/mktime.xml
index ffd25ad1d1..c9eb578748 100644
--- a/reference/datetime/functions/mktime.xml
+++ b/reference/datetime/functions/mktime.xml
@@ -51,7 +51,10 @@
      <term><parameter>hour</parameter></term>
      <listitem>
       <para>
-       The number of the hour.
+       The number of the hour relevant to the start of the day determined by
+       <parameter>month</parameter>, <parameter>day</parameter> and <parameter>year</parameter>.
+       Negative values reference the hour before midnight of the day in question.
+       Values greater than 23 reference the appropriate hour in the following day(s).
       </para>
      </listitem>
     </varlistentry>
@@ -59,7 +62,9 @@
      <term><parameter>minute</parameter></term>
      <listitem>
       <para>
-       The number of the minute.
+       The number of the minute relevant to the start of the <parameter>hour</parameter>.
+       Negative values reference the minute in the previous hour.
+       Values greater than 59 reference the appropriate minute in the following hour(s).
       </para>
      </listitem>
     </varlistentry>
@@ -67,7 +72,9 @@
      <term><parameter>second</parameter></term>
      <listitem>
       <para>
-       The number of seconds past the minute.
+       The number of seconds relevant to the start of the <parameter>minute</parameter>.
+       Negative values reference the second in the previous minute.
+       Values greater than 59 reference the appropriate second in the following minute(s).
       </para>
      </listitem>
     </varlistentry>
@@ -75,7 +82,10 @@
      <term><parameter>month</parameter></term>
      <listitem>
       <para>
-       The number of the month.
+       The number of the month relevant to the end of the previous year.
+       Values 1 to 12 reference the normal calendar months of the year in question.
+       Values less than 1 (including negative values) reference the months in the previous year in reverse order, so 0 is December, -1 is November, etc.
+       Values greater than 12 reference the appropriate month in the following year(s).
       </para>
      </listitem>
     </varlistentry>
@@ -83,7 +93,10 @@
      <term><parameter>day</parameter></term>
      <listitem>
       <para>
-       The number of the day.
+       The number of the day relevant to the end of the previous month.
+       Values 1 to 28, 29, 30 or 31 (depending upon the month) reference the normal days in the relevant month.
+       Values less than 1 (including negative values) reference the days in the previous month, so 0 is the last day of the previous month, -1 is the day before that, etc.
+       Values greater than the number of days in the relevant month reference the appropriate day in the following month(s).
       </para>
      </listitem>
     </varlistentry>