diff --git a/src/PhpSpreadsheet/Calculation/Functions.php b/src/PhpSpreadsheet/Calculation/Functions.php
index b334c0a1..21ac9e7c 100644
--- a/src/PhpSpreadsheet/Calculation/Functions.php
+++ b/src/PhpSpreadsheet/Calculation/Functions.php
@@ -15,12 +15,13 @@ class Functions
      */
     const M_2DIVPI = 0.63661977236758134307553505349006;
 
-    /** constants */
     const COMPATIBILITY_EXCEL = 'Excel';
     const COMPATIBILITY_GNUMERIC = 'Gnumeric';
     const COMPATIBILITY_OPENOFFICE = 'OpenOfficeCalc';
 
+    /** Use of RETURNDATE_PHP_NUMERIC is discouraged - not 32-bit Y2038-safe, no timezone. */
     const RETURNDATE_PHP_NUMERIC = 'P';
+    /** Use of RETURNDATE_UNIX_TIMESTAMP is discouraged - not 32-bit Y2038-safe, no timezone. */
     const RETURNDATE_UNIX_TIMESTAMP = 'P';
     const RETURNDATE_PHP_OBJECT = 'O';
     const RETURNDATE_PHP_DATETIME_OBJECT = 'O';
diff --git a/src/PhpSpreadsheet/Shared/Date.php b/src/PhpSpreadsheet/Shared/Date.php
index b755ec3a..886049e9 100644
--- a/src/PhpSpreadsheet/Shared/Date.php
+++ b/src/PhpSpreadsheet/Shared/Date.php
@@ -163,7 +163,7 @@ class Date
      * @param float|int $excelTimestamp MS Excel serialized date/time value
      * @param null|DateTimeZone|string $timeZone The timezone to assume for the Excel timestamp,
      *                                                                        if you don't want to treat it as a UTC value
-     *                                                                    Use the default (UST) unless you absolutely need a conversion
+     *                                                                    Use the default (UTC) unless you absolutely need a conversion
      *
      * @return DateTime PHP date/time object
      */
@@ -206,11 +206,13 @@ class Date
 
     /**
      * Convert a MS serialized datetime value from Excel to a unix timestamp.
+     * The use of Unix timestamps, and therefore this function, is discouraged.
+     * They are not Y2038-safe on a 32-bit system, and have no timezone info.
      *
      * @param float|int $excelTimestamp MS Excel serialized date/time value
      * @param null|DateTimeZone|string $timeZone The timezone to assume for the Excel timestamp,
      *                                                                        if you don't want to treat it as a UTC value
-     *                                                                    Use the default (UST) unless you absolutely need a conversion
+     *                                                                    Use the default (UTC) unless you absolutely need a conversion
      *
      * @return int Unix timetamp for this date/time
      */
@@ -223,7 +225,8 @@ class Date
     /**
      * Convert a date from PHP to an MS Excel serialized date/time value.
      *
-     * @param mixed $dateValue Unix Timestamp or PHP DateTime object or a string
+     * @param mixed $dateValue PHP DateTime object or a string - Unix timestamp is also permitted, but discouraged;
+     *    not Y2038-safe on a 32-bit system, and no timezone info
      *
      * @return bool|float Excel date/time value
      *                                  or boolean FALSE on failure
@@ -262,6 +265,8 @@ class Date
 
     /**
      * Convert a Unix timestamp to an MS Excel serialized date/time value.
+     * The use of Unix timestamps, and therefore this function, is discouraged.
+     * They are not Y2038-safe on a 32-bit system, and have no timezone info.
      *
      * @param int $unixTimestamp Unix Timestamp
      *