bpo-35054: Add more index entries for symbols. (GH-10064)

diff --git a/Doc/library/time.rst b/Doc/library/time.rst
index 56f972c..d76e089 100644
--- a/Doc/library/time.rst
+++ b/Doc/library/time.rst
@@ -369,6 +369,9 @@
       :pep:`475` for the rationale).
 
 
+.. index::
+   single: %; datetime format
+
 .. function:: strftime(format[, t])
 
    Convert a tuple or :class:`struct_time` representing a time as returned by
@@ -500,6 +503,9 @@
    it is 3.
 
 
+.. index::
+   single: %; datetime format
+
 .. function:: strptime(string[, format])
 
    Parse a string representing a time according to a format.  The return value