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/eval.lua

@@ -341,6 +341,7 @@ return {
     string={args=1},
     strlen={args=1},
     strpart={args={2, 4}},
+    strptime={args=2},
     strridx={args={2, 3}},
     strtrans={args=1},
     strwidth={args=1},

src/nvim/eval/funcs.c

@@ -10189,6 +10189,36 @@ static void f_strpart(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   rettv->vval.v_string = (char_u *)xmemdupz(p + n, (size_t)len);
 }
 
+// "strptime({format}, {timestring})" function
+static void f_strptime(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+  char fmt_buf[NUMBUFLEN];
+  char str_buf[NUMBUFLEN];
+
+  struct tm tmval = { 0 };
+  char *fmt = (char *)tv_get_string_buf(&argvars[0], fmt_buf);
+  char *str = (char *)tv_get_string_buf(&argvars[1], str_buf);
+
+  vimconv_T conv = {
+    .vc_type = CONV_NONE,
+  };
+  char_u *enc = enc_locale();
+  convert_setup(&conv, p_enc, enc);
+  if (conv.vc_type != CONV_NONE) {
+    fmt = (char *)string_convert(&conv, (char_u *)fmt, NULL);
+  }
+  if (fmt == NULL
+      || os_strptime(str, fmt, &tmval) == NULL
+      || (rettv->vval.v_number = mktime(&tmval)) == -1) {
+    rettv->vval.v_number = 0;
+  }
+  if (conv.vc_type != CONV_NONE) {
+    xfree(fmt);
+  }
+  convert_setup(&conv, NULL, NULL);
+  xfree(enc);
+}
+
 /*
  * "strridx()" function
  */

src/nvim/os/time.c

@@ -196,6 +196,22 @@ char *os_ctime(char *result, size_t result_len)
   return os_ctime_r(&rawtime, result, result_len);
 }
 
+/// Portable version of POSIX strptime()
+///
+/// @param str[in]  string to convert
+/// @param format[in]  format to parse "str"
+/// @param tm[out]  time representation of "str"
+/// @return Pointer to first unprocessed character or NULL
+char *os_strptime(const char *str, const char *format, struct tm *tm)
+  FUNC_ATTR_NONNULL_ALL
+{
+#ifdef HAVE_STRPTIME
+  return strptime(str, format, tm);
+#else
+  return NULL;
+#endif
+}
+
 /// Obtains the current Unix timestamp.
 ///
 /// @return Seconds since epoch.

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