mu4e-helpers: restore missing mu4e-get-time-date

It was in use in mu4e-headers-custom-markers, but somehow got lost back in 2021,
it seems. Restore it.

Fixes #2879.

mu4e/mu4e-headers.el

@@ -268,7 +268,7 @@ The first character of NAME is used as the shortcut."
      (lambda () (mu4e-get-time-date "Match messages after: ")))
     ("Bigger than"
      (lambda (msg bytes) (> (mu4e-msg-field msg :size) (* 1024 bytes)))
-     (lambda () (read-number "Match messages bigger than (Kbytes): "))))
+     (lambda () (read-number "Match messages bigger than (KB): "))))
   "List of custom markers-functions.
 
 The functions are to mark message that match some custom
@@ -281,7 +281,11 @@ and (optionally) PARAM, and should return non-nil when there's a
 match.
 * PARAM-FUNC is function that is evaluated once, and its value is
 then passed to PREDICATE-FUNC as PARAM. This is useful for
-getting user-input.")
+getting user-input.
+
+The default functions that take dates, use `mu4e-get-time-date'
+which wraps `parse-time-string' and hence requires the format to
+be compatible with that.")
 ;;; Internal variables/constants
 
 ;; docid cookies

mu4e/mu4e-helpers.el

@@ -501,6 +501,13 @@ This includes expanding e.g. 3-5 into 3,4,5. If the letter
          (mu4e-warn "Attachment number must be greater than 0 (%d)" x))))
      list)))
 
+(defun mu4e-get-time-date (prompt)
+  "Determine the Emacs time value for time/date entered by user.
+Basically, a command around `parse-time-string' to read a user
+string after PROMPT."
+  (let ((timestr (read-string (mu4e-format "%s" prompt))))
+    (apply 'encode-time (parse-time-string timestr))))
+
 (defun mu4e-make-temp-file (ext)
   "Create a self-destructing temporary file with extension EXT.
 The file will self-destruct in a short while, enough to open it