diff --git a/core/src/Xios.cpp b/core/src/Xios.cpp
index 274640cfa..93b65a176 100644
--- a/core/src/Xios.cpp
+++ b/core/src/Xios.cpp
@@ -49,13 +49,16 @@ void enableXios()
  *
  * @param dt Timestep to use for the model
  * @param contextid ID for the XIOS context
+ * @param starttime Datetime string for the start of the simulation
  * @param calendartype Type of calendar to use
  */
-Xios::Xios(const std::string dt, const std::string contextid, const std::string calendartype)
+Xios::Xios(const std::string dt, const std::string contextid, const std::string starttime,
+    const std::string calendartype)
 {
-    calendarType = calendartype;
-    contextId = contextid;
     timestep = Duration(dt);
+    startTime = TimePoint(starttime);
+    contextId = contextid;
+    calendarType = calendartype;
     configure();
 }
 
@@ -127,7 +130,7 @@ void Xios::configureServer()
 
     // Set default calendar origin and start
     setCalendarOrigin(TimePoint("1970-01-01T00:00:00Z")); // Unix epoch
-    setCalendarStart(TimePoint("2023-03-17T17:11:00Z")); // TODO: Read from config
+    setCalendarStart(TimePoint(startTime));
 }
 
 /*!
diff --git a/core/src/include/Xios.hpp b/core/src/include/Xios.hpp
index 7b91e007c..cd846ded7 100644
--- a/core/src/include/Xios.hpp
+++ b/core/src/include/Xios.hpp
@@ -32,6 +32,7 @@ void enableXios();
 class Xios : public Configured<Xios> {
 public:
     Xios(const std::string dt = "P0-0T01:00:00", const std::string contextid = "nextSIM-DG",
+        const std::string starttime = "1970-01-01T00:00:00Z",
         const std::string calendartype = "Gregorian");
     ~Xios();
 
@@ -150,6 +151,7 @@ class Xios : public Configured<Xios> {
     std::string calendarType;
     std::string contextId;
     Duration timestep;
+    TimePoint startTime;
     MPI_Comm clientComm;
     MPI_Fint clientComm_F;
     MPI_Fint nullComm_F;
diff --git a/core/test/XiosCalendar_test.cpp b/core/test/XiosCalendar_test.cpp
index a77699789..6dcd0a7cc 100644
--- a/core/test/XiosCalendar_test.cpp
+++ b/core/test/XiosCalendar_test.cpp
@@ -42,8 +42,9 @@ MPI_TEST_CASE("TestXiosInitialization", 2)
     TimePoint origin("2020-01-23T00:08:15Z");
     xios_handler.setCalendarOrigin(origin);
     REQUIRE(origin == xios_handler.getCalendarOrigin());
-    REQUIRE(origin.format() == "1970-01-01T00:00:00Z");
+    REQUIRE(origin.format() == "2020-01-23T00:08:15Z");
     // Calendar start
+    REQUIRE(xios_handler.getCalendarStart().format() == "1970-01-01T00:00:00Z"); // Default
     TimePoint start("2023-03-17T17:11:00Z");
     xios_handler.setCalendarStart(start);
     REQUIRE(start == xios_handler.getCalendarStart());
diff --git a/core/test/XiosReadWrite_test.cpp b/core/test/XiosReadWrite_test.cpp
index ab497e10b..d746216a8 100644
--- a/core/test/XiosReadWrite_test.cpp
+++ b/core/test/XiosReadWrite_test.cpp
@@ -1,7 +1,7 @@
 /*!
  * @file    XiosReadWrite_test.cpp
  * @author  Joe Wallwork <jw2423@cam.ac.uk>
- * @date    03 Dec 2024
+ * @date    04 Dec 2024
  * @brief   Tests for XIOS write method
  * @details
  * This test is designed to test the read and write methods of the C++
@@ -61,7 +61,7 @@ Xios setupXiosHandler(int dim, bool read)
     } else {
         label = "write";
     }
-    Xios xios_handler("P0-0T01:30:00", formatId(label, dim));
+    Xios xios_handler("P0-0T01:30:00", formatId(label, dim), "2023-03-17T17:11:00Z");
     REQUIRE(xios_handler.isInitialized());
     const size_t size = xios_handler.getClientMPISize();
     REQUIRE(size == 2);