[flang] Update the date_and_time intrinsic for AIX (#104849)

Currently, strftime is called to get the timezone for the ZONE argument.
On AIX, this routine requires an environment variable set in order to 
return the required format. This patch is to add the time difference 
computation from UTC for the platform.

Author: kkwli

flang/runtime/time-intrinsic.cpp

@@ -247,6 +247,61 @@ static void DateAndTimeUnavailable(Fortran::runtime::Terminator &terminator,
 }
 
 #ifndef _WIN32
+#ifdef _AIX
+// Compute the time difference from GMT/UTC to get around the behavior of
+// strfname on AIX that requires setting an environment variable for numeric
+// value for ZONE.
+// The ZONE and the VALUES(4) arguments of the DATE_AND_TIME intrinsic has
+// the resolution to the minute.
+static int computeUTCDiff(const tm &localTime, bool *err) {
+  tm utcTime;
+  const time_t timer{mktime(const_cast<tm *>(&localTime))};
+  if (timer < 0) {
+    *err = true;
+    return 0;
+  }
+
+  // Get the GMT/UTC time
+  if (gmtime_r(&timer, &utcTime) == nullptr) {
+    *err = true;
+    return 0;
+  }
+
+  // Adjust for day difference
+  auto dayDiff{localTime.tm_mday - utcTime.tm_mday};
+  auto localHr{localTime.tm_hour};
+  if (dayDiff > 0) {
+    if (dayDiff == 1) {
+      localHr += 24;
+    } else {
+      utcTime.tm_hour += 24;
+    }
+  } else if (dayDiff < 0) {
+    if (dayDiff == -1) {
+      utcTime.tm_hour += 24;
+    } else {
+      localHr += 24;
+    }
+  }
+  return (localHr * 60 + localTime.tm_min) -
+      (utcTime.tm_hour * 60 + utcTime.tm_min);
+}
+#endif
+
+static std::size_t getUTCOffsetToBuffer(
+    char *buffer, const std::size_t &buffSize, tm *localTime) {
+#ifdef _AIX
+  // format: +HHMM or -HHMM
+  bool err{false};
+  auto utcOffset{computeUTCDiff(*localTime, &err)};
+  auto hour{utcOffset / 60};
+  auto hrMin{hour * 100 + (utcOffset - hour * 60)};
+  auto n{sprintf(buffer, "%+05d", hrMin)};
+  return err ? 0 : n + 1;
+#else
+  return std::strftime(buffer, buffSize, "%z", localTime);
+#endif
+}
 
 // SFINAE helper to return the struct tm.tm_gmtoff which is not a POSIX standard
 // field.
@@ -263,8 +318,19 @@ GetGmtOffset(const TM &tm, fallback_implementation) {
   // tm.tm_gmtoff is not available, there may be platform dependent alternatives
   // (such as using timezone from <time.h> when available), but so far just
   // return -HUGE to report that this information is not available.
-  return -std::numeric_limits<Fortran::runtime::CppTypeFor<
-      Fortran::common::TypeCategory::Integer, KIND>>::max();
+  const auto negHuge{-std::numeric_limits<Fortran::runtime::CppTypeFor<
+      Fortran::common::TypeCategory::Integer, KIND>>::max()};
+#ifdef _AIX
+  bool err{false};
+  auto diff{computeUTCDiff(tm, &err)};
+  if (err) {
+    return negHuge;
+  } else {
+    return diff;
+  }
+#else
+  return negHuge;
+#endif
 }
 template <typename TM = struct tm> struct GmtOffsetHelper {
   template <int KIND> struct StoreGmtOffset {
@@ -317,7 +383,7 @@ static void GetDateAndTime(Fortran::runtime::Terminator &terminator, char *date,
     // Note: this may leave the buffer empty on many platforms. Classic flang
     // has a much more complex way of doing this (see __io_timezone in classic
     // flang).
-    auto len{std::strftime(buffer, buffSize, "%z", &localTime)};
+    auto len{getUTCOffsetToBuffer(buffer, buffSize, &localTime)};
     copyBufferAndPad(zone, zoneChars, len);
   }
   if (values) {