diff --git a/core/CMakeLists.txt b/core/CMakeLists.txt
index 1c63b333..fee17915 100644
--- a/core/CMakeLists.txt
+++ b/core/CMakeLists.txt
@@ -113,6 +113,15 @@ if (USE_DAMARIS)
     SET(ross_srcs ${ross_srcs} ${ROSS_Damaris_SOURCE_DIR}/core/damaris.h)
 ENDIF(USE_DAMARIS)
 
+OPTION(USE_EXTERNAL_STIME "Use model-specific tw_stime implementation" OFF)
+IF(USE_EXTERNAL_STIME)
+  #SET(ross_srcs ${ross_srcs} ${MODEL_STIME_SRC} ${MODEL_STIME_HDR})
+  #GET_FILENAME_COMPONENT(${stime_header_path} ${MODEL_STIME_HDR} DIRECTORY)
+  #INCLUDE_DIRECTORIES(${stime_header_path})
+  SET(ross_srcs ${ross_srcs} ${MODEL_STIME_SRC})
+  INCLUDE_DIRECTORIES(${MODEL_STIME_HDR})
+ENDIF(USE_EXTERNAL_STIME)
+
 # Use debugging-friendly memory allocation
 OPTION(ROSS_ALLOC_DEBUG "Use naive allocator to be more friendly to memory debugging tools" OFF)
 
diff --git a/core/config.h.in b/core/config.h.in
index 02d04e96..d317c615 100644
--- a/core/config.h.in
+++ b/core/config.h.in
@@ -17,3 +17,8 @@
 #cmakedefine ROSS_ALLOC_DEBUG
 #cmakedefine USE_RIO
 #cmakedefine USE_DAMARIS
+
+#cmakedefine USE_EXTERNAL_STIME
+#ifdef USE_EXTERNAL_STIME
+#include "${MODEL_STIME_HDR}"
+#endif
diff --git a/core/gvt/mpi_allreduce.c b/core/gvt/mpi_allreduce.c
index a3cc499d..2ac48952 100644
--- a/core/gvt/mpi_allreduce.c
+++ b/core/gvt/mpi_allreduce.c
@@ -107,8 +107,9 @@ tw_gvt_step2(tw_pe *me)
 	tw_stime pq_min = TW_STIME_MAX;
 	tw_stime net_min = TW_STIME_MAX;
 
-	tw_stime lvt;
-	tw_stime gvt;
+
+        tw_stime lvt;
+        tw_stime gvt;
 
     tw_clock net_start;
 	tw_clock start = tw_clock_read();
@@ -148,15 +149,27 @@ tw_gvt_step2(tw_pe *me)
 
 	all_reduce_cnt++;
 
+        // Note for TW_STIME API
+        // lvt is cast to double when going into the allreduce
+        // thus, the gvt that comes out is a double
+        // and needs to be cast back to an tw_stime object
+        //
+        // must continue to use MPI_DOUBLE type... or reimplement allreduce
+
+        double lvtd = TW_STIME_DBL(lvt);
+        double gvtd;
+
 	if(MPI_Allreduce(
-			&lvt,
-			&gvt,
+			&lvtd,
+			&gvtd,
 			1,
-			MPI_TYPE_TW_STIME,
+			MPI_DOUBLE,
 			MPI_MIN,
 			MPI_COMM_ROSS) != MPI_SUCCESS)
 			tw_error(TW_LOC, "MPI_Allreduce for GVT failed");
 
+        gvt = TW_STIME_CRT(gvtd);
+
 	if(TW_STIME_CMP(gvt, me->GVT_prev) < 0)
 	{
 		g_tw_gvt_no_change = 0;
diff --git a/core/ross.h b/core/ross.h
index 978e3074..ecedd4cb 100644
--- a/core/ross.h
+++ b/core/ross.h
@@ -141,12 +141,12 @@ extern "C" {
 #  define free(b) must_not_use_free
 #endif
 
-// #include "config.h" -- moved to individual files that need them -- e.g., tw-setup.c
-
 /* tw_peid -- Processing Element "PE" id */
 typedef unsigned long tw_peid;
 
 /* tw_stime -- Simulation time value for sim clock (NOT wall!) */
+/* Model stime header included by config.h */
+#ifndef USE_EXTERNAL_STIME
 typedef double tw_stime;
 #define MPI_TYPE_TW_STIME   MPI_DOUBLE
 #define TW_STIME_CRT(x)     (x)
@@ -154,6 +154,7 @@ typedef double tw_stime;
 #define TW_STIME_CMP(x, y)  (((x) < (y)) ? -1 : ((x) > (y)))
 #define TW_STIME_ADD(x, y)  ((x) + (y))
 #define TW_STIME_MAX        DBL_MAX
+#endif
 
 /* tw_lpid -- Logical Process "LP" id */
 //typedef unsigned long long tw_lpid;
diff --git a/core/tw-opts.c b/core/tw-opts.c
index 3a5dce64..d28e525f 100644
--- a/core/tw-opts.c
+++ b/core/tw-opts.c
@@ -58,9 +58,11 @@ show_help(void)
 				pos += fprintf(stderr, "=dbl");
                                 break;
 
+#ifndef USE_EXTERNAL_STIME
 			case TWOPTTYPE_STIME:
 				pos += fprintf(stderr, "=ts");
 				break;
+#endif
 
 			case TWOPTTYPE_CHAR:
 				pos += fprintf(stderr, "=str");
@@ -104,9 +106,11 @@ show_help(void)
 					fprintf(stderr, " (default %.2f)", *((double*)def->value));
                                         break;
 
+#ifndef USE_EXTERNAL_STIME
 				case TWOPTTYPE_STIME:
-					fprintf(stderr, " (default %.2f)", *((tw_stime*)def->value));
+                                    fprintf(stderr, " (default %.2f)", *((tw_stime*)def->value));
 					break;
+#endif
 
 				case TWOPTTYPE_CHAR:
 					fprintf(stderr, " (default %s)", (char *) def->value);
@@ -184,9 +188,11 @@ void tw_opt_settings(FILE *outfile) {
                     fprintf(outfile, "%.2f", *((double*)def->value));
                     break;
 
+#ifndef USE_EXTERNAL_STIME
                 case TWOPTTYPE_STIME:
                     fprintf(outfile, "%.2f", *((tw_stime*)def->value));
                     break;
+#endif
 
                 case TWOPTTYPE_CHAR:
                     fprintf(outfile, "%s", (char *) def->value);
@@ -249,9 +255,11 @@ tw_opt_print(void)
 					fprintf(f, "%.2f,", *((double*)def->value));
                                         break;
 
+#ifndef USE_EXTERNAL_STIME
 				case TWOPTTYPE_STIME:
-					fprintf(f, "%.2f,", *((tw_stime*)def->value));
+                                        fprintf(f, "%.2f,", *((tw_stime*)def->value));
 					break;
+#endif
 
 				case TWOPTTYPE_CHAR:
 					fprintf(f, "%s,", (char *)def->value);
@@ -332,6 +340,7 @@ apply_opt(const tw_optdef *def, const char *value)
 		break;
 	}
 
+#ifndef USE_EXTERNAL_STIME
 	case TWOPTTYPE_STIME:
 	{
 		tw_stime v;
@@ -347,6 +356,7 @@ apply_opt(const tw_optdef *def, const char *value)
 		*((tw_stime*)def->value) = v;
 		break;
 	}
+#endif
 
 	case TWOPTTYPE_CHAR:
 	{
diff --git a/core/tw-opts.h b/core/tw-opts.h
index e7af1b85..601e01b4 100644
--- a/core/tw-opts.h
+++ b/core/tw-opts.h
@@ -7,7 +7,9 @@ enum tw_opttype
 	TWOPTTYPE_ULONG,       /**< value must be an "unsigned long*"      */
 	TWOPTTYPE_ULONGLONG,   /**< value must be an "unsigned long long*" */
 	TWOPTTYPE_UINT,        /**< value must be an "unsigned int*"       */
+#ifndef USE_EXTERNAL_STIME
 	TWOPTTYPE_STIME,       /**< value must be a  "tw_stime*"           */
+#endif
         TWOPTTYPE_DOUBLE,      /**< value must be a  "double *"            */
 	TWOPTTYPE_CHAR,        /**< value must be a  "char *"              */
         TWOPTTYPE_FLAG,        /**< value must be an "unsigned int*"       */
@@ -28,7 +30,9 @@ struct tw_optdef
 #define TWOPT_ULONG(n,v,h)     { TWOPTTYPE_ULONG,     (n), (h), &(v) }
 #define TWOPT_ULONGLONG(n,v,h) { TWOPTTYPE_ULONGLONG, (n), (h), &(v) }
 #define TWOPT_UINT(n,v,h)      { TWOPTTYPE_UINT,      (n), (h), &(v) }
-#define TWOPT_STIME(n,v,h)     { TWOPTTYPE_STIME,     (n), (h), &(v) }
+#ifndef USE_EXTERNAL_STIME
+  #define TWOPT_STIME(n,v,h)     { TWOPTTYPE_STIME,     (n), (h), &(v) }
+#endif
 #define TWOPT_DOUBLE(n,v,h)    { TWOPTTYPE_DOUBLE,    (n), (h), &(v) }
 #define TWOPT_CHAR(n,v,h)      { TWOPTTYPE_CHAR,      (n), (h), &(v) }
 #define TWOPT_FLAG(n,v,h)      { TWOPTTYPE_FLAG,      (n), (h), &(v) }
diff --git a/core/tw-pe.c b/core/tw-pe.c
index a974debc..3064bf72 100644
--- a/core/tw-pe.c
+++ b/core/tw-pe.c
@@ -41,7 +41,7 @@ tw_pe_init(void)
     memset(&no_type, 0, sizeof(no_type));
     tw_pe_settype(&no_type);
 
-    g_tw_pe->trans_msg_ts = DBL_MAX;
+    g_tw_pe->trans_msg_ts = TW_STIME_MAX;
     g_tw_pe->gvt_status = 0;
 
     // TODO is the PE RNG ever actually used?