diff --git a/src/rpc/misc.cpp b/src/rpc/misc.cpp
--- a/src/rpc/misc.cpp
+++ b/src/rpc/misc.cpp
@@ -330,7 +330,12 @@
     LOCK(cs_main);
 
     RPCTypeCheck(request.params, {UniValue::VNUM});
-    SetMockTime(request.params[0].get_int64());
+    int64_t mockTime = request.params[0].get_int64();
+    if (mockTime < 0) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER,
+                           "Timestamp must be 0 or greater");
+    }
+    SetMockTime(mockTime);
 
     return NullUniValue;
 }
diff --git a/src/util/time.cpp b/src/util/time.cpp
--- a/src/util/time.cpp
+++ b/src/util/time.cpp
@@ -44,6 +44,7 @@
 template std::chrono::microseconds GetTime();
 
 void SetMockTime(int64_t nMockTimeIn) {
+    assert(nMockTimeIn >= 0);
     nMockTime.store(nMockTimeIn, std::memory_order_relaxed);
 }
 
diff --git a/test/functional/abc-rpc-mocktime.py b/test/functional/abc-rpc-mocktime.py
new file mode 100755
--- /dev/null
+++ b/test/functional/abc-rpc-mocktime.py
@@ -0,0 +1,28 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 The Bitcoin developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""
+Test RPCs related to mock time.
+"""
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import assert_raises_rpc_error
+
+
+class MocktimeTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+        self.setup_clean_chain = True
+
+    def run_test(self):
+        self.nodes[0].setmocktime(9223372036854775807)
+        self.nodes[0].setmocktime(0)
+        assert_raises_rpc_error(-8, "Timestamp must be 0 or greater",
+                                self.nodes[0].setmocktime, -1)
+        assert_raises_rpc_error(-8, "Timestamp must be 0 or greater",
+                                self.nodes[0].setmocktime, -9223372036854775808)
+
+
+if __name__ == '__main__':
+    MocktimeTest().main()