fix: use timegm/_mkgmtime instead of std::mktime for permission timestamps

std::mktime interprets the broken-down time as local time, so permission
validity periods change depending on the timezone of the host parsing the
XML. This breaks deployments where permissions documents are distributed
across hosts in different timezones.

Replace std::mktime with timegm (POSIX) / _mkgmtime (Windows) so the
not_before and not_after timestamps are always interpreted as UTC.

Fixes #6403.

Test Plan:
Parsed a permissions XML with validity="2025-01-01T00:00:00" and
verified the epoch time is the same regardless of TZ environment
variable.

Signed-off-by: Yuchen Fan <functionhx@gmail.com>

Author: Functionhx

src/cpp/security/accesscontrol/PermissionsParser.cpp

@@ -20,6 +20,7 @@
 #include <iostream>
 #include <sstream>
 #include <iomanip>
+#include <ctime>
 
 #if TIXML2_MAJOR_VERSION >= 6
 #define PRINTLINE(node) node->GetLineNum()
@@ -339,7 +340,11 @@ bool PermissionsParser::parse_validity(
 
                 if (!stream.fail())
                 {
-                    validity.not_before = std::mktime(&time);
+#ifdef _WIN32
+                    validity.not_before = _mkgmtime(&time);
+#else
+                    validity.not_before = timegm(&time);
+#endif // ifdef _WIN32
 #endif // if _MSC_VER != 1800
 
                 tinyxml2::XMLElement* old_node = node;
@@ -360,7 +365,11 @@ bool PermissionsParser::parse_validity(
 
                             if (!stream.fail())
                             {
-                                validity.not_after = std::mktime(&time);
+#ifdef _WIN32
+                                validity.not_after = _mkgmtime(&time);
+#else
+                                validity.not_after = timegm(&time);
+#endif // ifdef _WIN32
 #endif // if _MSC_VER != 1800
                             returned_value = true;
                         }