Improve date-time parsing

@punchagan

README.org

@@ -278,6 +278,11 @@ have the exported Markdown files.
   front-matter [[[https://github.com/kaushalmodi/ox-hugo/issues/68][68]]].
 - Code optimization: Use of =org-entry-get= at places instead of
   maintaining global variables.
+** 0.1.2
+- Make DateTime matching better; new internal variable
+  =org-hugo--date-time-regexp=. Earlier time zones ahead of UTC (with
+  =+= sign) were not detected as dates in =org-hugo--quote-string= and
+  thus were unnecessarily quoted.
 * Debug
 If the =ox-hugo= exports do not work as expected, or if you get an
 error backtrace,

doc/ox-hugo-manual.org

@@ -29,6 +29,11 @@ Markdown]] flavor.
   front-matter {{{issue(68)}}}.
 - Code optimization: Use of =org-entry-get= at places instead of
   maintaining global variables.
+** 0.1.2
+- Make DateTime matching better; new internal variable
+  =org-hugo--date-time-regexp=. Earlier time zones ahead of UTC (with
+  =+= sign) were not detected as dates in =org-hugo--quote-string= and
+  thus were unnecessarily quoted.
 * Note to Future Contributors
 :PROPERTIES:
 :CUSTOM_ID: note-to-future-contributors

example-site/content-org/all-posts.org

@@ -1384,6 +1384,32 @@ If such a property is non-nil, the value (time-stamp) of that is used
 to set the =date= field in the exported front-matter.
 
 - Reference :: [[http://orgmode.org/manual/Special-properties.html][(org) Special properties]] or =C-h i g (org) Special properties=
+* Date Formats                                                         :date:
+** Just date
+:PROPERTIES:
+:EXPORT_FILE_NAME: date-just-date
+:EXPORT_DATE: 2017-09-12
+:END:
+** Date + Time
+:PROPERTIES:
+:EXPORT_FILE_NAME: date-plus-time
+:EXPORT_DATE: 2017-09-12T16:10:00
+:END:
+** Date + Time (UTC)
+:PROPERTIES:
+:EXPORT_FILE_NAME: date-plus-time-utc
+:EXPORT_DATE: 2017-09-12T16:10:00Z
+:END:
+** Date + Time (behind UTC)
+:PROPERTIES:
+:EXPORT_FILE_NAME: date-plus-time-minus-utc
+:EXPORT_DATE: 2017-09-12T16:10:00-04:00
+:END:
+** Date + Time (after UTC)
+:PROPERTIES:
+:EXPORT_FILE_NAME: date-plus-time-plus-utc
+:EXPORT_DATE: 2017-09-12T16:10:00+05:30
+:END:
 * TODO Pre-Draft State
 :PROPERTIES:
 :EXPORT_FILE_NAME: draft-state-todo

example-site/content/posts/date-just-date.md

@@ -0,0 +1,6 @@
++++
+title = "Just date"
+date = 2017-09-12
+tags = ["date"]
+draft = false
++++

example-site/content/posts/date-plus-time-minus-utc.md

@@ -0,0 +1,6 @@
++++
+title = "Date + Time (behind UTC)"
+date = 2017-09-12T16:10:00-04:00
+tags = ["date"]
+draft = false
++++

example-site/content/posts/date-plus-time-plus-utc.md

@@ -0,0 +1,6 @@
++++
+title = "Date + Time (after UTC)"
+date = 2017-09-12T16:10:00+05:30
+tags = ["date"]
+draft = false
++++

example-site/content/posts/date-plus-time-utc.md

@@ -0,0 +1,6 @@
++++
+title = "Date + Time (UTC)"
+date = 2017-09-12T16:10:00Z
+tags = ["date"]
+draft = false
++++

example-site/content/posts/date-plus-time.md

@@ -0,0 +1,6 @@
++++
+title = "Date + Time"
+date = 2017-09-12T16:10:00
+tags = ["date"]
+draft = false
++++

ox-hugo.el

@@ -5,7 +5,7 @@
 ;; URL: https://github.com/kaushalmodi/ox-hugo
 ;; Package-Requires: ((emacs "24.5") (org "9.0"))
 ;; Keywords: Org, markdown, docs
-;; Version: 0.1.1
+;; Version: 0.1.2
 
 ;;; Commentary:
 
@@ -299,6 +299,20 @@ joinLines
 This variable is for internal use only, and must not be
 modified.")
 
+(defvar org-hugo--date-time-regexp (concat "\\`[[:digit:]]\\{4\\}-[[:digit:]]\\{2\\}-[[:digit:]]\\{2\\}"
+                                           "\\(?:T[[:digit:]]\\{2\\}:[[:digit:]]\\{2\\}:[[:digit:]]\\{2\\}"
+                                           "\\(?:Z\\|[+-][[:digit:]]\\{2\\}:[[:digit:]]\\{2\\}\\)*\\)*\\'")
+  "Regexp to match the Hugo time stamp strings.
+
+Reference: https://tools.ietf.org/html/rfc3339#section-5.8
+
+Examples:
+  2017-07-31
+  2017-07-31T17:05:38
+  2017-07-31T17:05:38Z
+  2017-07-31T17:05:38+04:00
+  2017-07-31T17:05:38-04:00.")
+
 
 ;;; User-Configurable Variables
 
@@ -884,7 +898,9 @@ string with just alphanumeric characters."
         (string= "true" val)
         (string= "false" val)
         ;; or if it is a date (date, publishDate, expiryDate, lastmod)
-        (string-match-p "\\`[-0-9T:]+\\'" val))
+        (string-match-p org-hugo--date-time-regexp val)
+        ;; or if it is any number
+        (string-match-p "\\`[[:digit:]]+\\'" val))
     val)
    ((and prefer-no-quotes
          (string-match-p "\\`[a-zA-Z0-9]+\\'" val))
@@ -1116,7 +1132,7 @@ INFO is a plist used as a communication channel."
                     ;; the Org file.
                     (org-string-nw-p (org-export-get-date info hugo-date-fmt))))
          (date-nocolon (and (stringp date-raw)
-                            (if (string-match-p "\\`[0-9T:-]+\\'" date-raw)
+                            (if (string-match-p org-hugo--date-time-regexp date-raw)
                                 ;; If the set DATE is already in
                                 ;; Hugo-compatible date format, use it.
                                 date-raw
@@ -1137,7 +1153,7 @@ INFO is a plist used as a communication channel."
                            ;; If the set HUGO_LASTMOD is already in
                            ;; Hugo-compatible lastmod format, use it.
                            ((and (stringp lastmod-raw)
-                                 (string-match-p "\\`[0-9T:-]+\\'" lastmod-raw))
+                                 (string-match-p org-hugo--date-time-regexp lastmod-raw))
                             lastmod-raw)
                            ;; Else if it's a string, try to parse the lastmod.
                            ((stringp lastmod-raw)