vim-patch:8.1.2326: cannot parse a date/time string

Problem:    Cannot parse a date/time string.
Solution:   Add strptime(). (Stephen Wall, closes #)
10455d43fe

N/A patches for version.c:

vim-patch:8.1.2344: Cygwin: warning for using strptime()

Problem:    Cygwin: warning for using strptime().
Solution:   Move defining _XOPEN_SOURCE and __USE_XOPEN to vim.h. (Ken Takata,
            closes vim/vim#5265)  Use 700 for _XOPEN_SOURCE for mkdtemp().
6a228c6463

src/nvim/testdir/test_functions.vim

@@ -1,5 +1,7 @@
 " Tests for various functions.
+
 source shared.vim
+source check.vim
 
 " Must be done first, since the alternate buffer must be unset.
 func Test_00_bufexists()
@@ -171,9 +173,8 @@ func Test_str2nr()
 endfunc
 
 func Test_strftime()
-  if !exists('*strftime')
-    return
-  endif
+  CheckFunction strftime
+
   " Format of strftime() depends on system. We assume
   " that basic formats tested here are available and
   " identical on all systems which support strftime().
@@ -214,6 +215,28 @@ func Test_strftime()
   endif
 endfunc
 
+func Test_strptime()
+  CheckFunction strptime
+
+  if exists('$TZ')
+    let tz = $TZ
+  endif
+  let $TZ = 'UTC'
+
+  call assert_equal(1484653763, strptime('%Y-%m-%d %X', '2017-01-17 11:49:23'))
+
+  call assert_fails('call strptime()', 'E119:')
+  call assert_fails('call strptime("xxx")', 'E119:')
+  call assert_equal(0, strptime("%Y", ''))
+  call assert_equal(0, strptime("%Y", "xxx"))
+
+  if exists('tz')
+    let $TZ = tz
+  else
+    unlet $TZ
+  endif
+endfunc
+
 func Test_resolve_unix()
   if !has('unix')
     return