fix the year 2038 problem in getDateTime

The existing codes cast uint64_t into time_t which is int32_t in
most 32-bit systems. It results overflow if the timestamp is larger
than INT_MAX.
time_t will be 64 bits in future releases of glibc. See
https://sourceware.org/bugzilla/show_bug.cgi?id=28182.

This change workarounds the year 2038 problem via boost's ptime.
std::chrono doesn't help since it is still 32 bits.

Tested on QEMU.
Example output for certificate:
{
  "Name": "HTTPS Certificate",
  "Subject": null,
  "ValidNotAfter": "2106-01-28T20:40:31Z",
  "ValidNotBefore": "2106-02-06T18:28:16Z"
}
Previously, the format is like "1969-12-31T12:00:00+00:00". Note
that the ending "+00:00" is the time zone, not ms.

Tested the schema on QEMU. No new Redfish Service Validator errors.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: I8ef0bee3d724184d96253c23f3919447828d3f82
diff --git a/redfish-core/lib/certificate_service.hpp b/redfish-core/lib/certificate_service.hpp
index 045426e..8a78317 100644
--- a/redfish-core/lib/certificate_service.hpp
+++ b/redfish-core/lib/certificate_service.hpp
@@ -643,9 +643,8 @@
                         std::get_if<uint64_t>(&property.second);
                     if (value)
                     {
-                        std::time_t time = static_cast<std::time_t>(*value);
                         asyncResp->res.jsonValue["ValidNotAfter"] =
-                            crow::utility::getDateTime(time);
+                            crow::utility::getDateTimeUint(*value);
                     }
                 }
                 else if (property.first == "ValidNotBefore")
@@ -654,9 +653,8 @@
                         std::get_if<uint64_t>(&property.second);
                     if (value)
                     {
-                        std::time_t time = static_cast<std::time_t>(*value);
                         asyncResp->res.jsonValue["ValidNotBefore"] =
-                            crow::utility::getDateTime(time);
+                            crow::utility::getDateTimeUint(*value);
                     }
                 }
             }