diff --git a/src/net_processing.cpp b/src/net_processing.cpp
--- a/src/net_processing.cpp
+++ b/src/net_processing.cpp
@@ -3464,6 +3464,9 @@
         uint64_t nExtraEntropy = 1;
 
         vRecv >> nVersion >> nServiceInt >> nTime >> addrMe;
+        if (nTime < 0) {
+            nTime = 0;
+        }
         nServices = ServiceFlags(nServiceInt);
         if (!pfrom.IsInboundConn()) {
             m_connman.SetServices(pfrom.addr, nServices);
diff --git a/src/rpc/misc.cpp b/src/rpc/misc.cpp
--- a/src/rpc/misc.cpp
+++ b/src/rpc/misc.cpp
@@ -445,9 +445,8 @@
         "Set the local time to given timestamp (-regtest only)\n",
         {
             {"timestamp", RPCArg::Type::NUM, RPCArg::Optional::NO,
-             UNIX_EPOCH_TIME +
-                 "\n"
-                 "   Pass 0 to go back to using the system time."},
+             UNIX_EPOCH_TIME + "\n"
+                               "Pass 0 to go back to using the system time."},
         },
         RPCResult{RPCResult::Type::NONE, "", ""},
         RPCExamples{""},
@@ -466,10 +465,11 @@
             LOCK(cs_main);
 
             RPCTypeCheck(request.params, {UniValue::VNUM});
-            int64_t time = request.params[0].get_int64();
+            const int64_t time{request.params[0].get_int64()};
             if (time < 0) {
-                throw JSONRPCError(RPC_INVALID_PARAMETER,
-                                   "Timestamp must be 0 or greater");
+                throw JSONRPCError(
+                    RPC_INVALID_PARAMETER,
+                    strprintf("Mocktime can not be negative: %s.", time));
             }
             SetMockTime(time);
             auto node_context = util::AnyPtr<NodeContext>(request.context);
diff --git a/src/util/time.cpp b/src/util/time.cpp
--- a/src/util/time.cpp
+++ b/src/util/time.cpp
@@ -3,12 +3,14 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <util/time.h>
+
 #if defined(HAVE_CONFIG_H)
 #include <config/bitcoin-config.h>
 #endif
 
 #include <compat.h>
-#include <util/time.h>
+#include <util/check.h>
 
 #include <boost/date_time/posix_time/posix_time.hpp>
 
@@ -22,7 +24,7 @@
     std::this_thread::sleep_for(n);
 }
 
-//! For unit testing
+//! For testing
 static std::atomic<int64_t> nMockTime(0);
 
 int64_t GetTime() {
@@ -49,7 +51,7 @@
 template std::chrono::microseconds GetTime();
 
 void SetMockTime(int64_t nMockTimeIn) {
-    assert(nMockTimeIn >= 0);
+    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
deleted file mode 100755
--- a/test/functional/abc_rpc_mocktime.py
+++ /dev/null
@@ -1,28 +0,0 @@
-#!/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()
diff --git a/test/functional/rpc_uptime.py b/test/functional/rpc_uptime.py
--- a/test/functional/rpc_uptime.py
+++ b/test/functional/rpc_uptime.py
@@ -10,6 +10,7 @@
 import time
 
 from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import assert_raises_rpc_error
 
 
 class UptimeTest(BitcoinTestFramework):
@@ -18,8 +19,19 @@
         self.setup_clean_chain = True
 
     def run_test(self):
+        self._test_negative_time()
         self._test_uptime()
 
+    def _test_negative_time(self):
+        self.nodes[0].setmocktime(9223372036854775807)
+        self.nodes[0].setmocktime(0)
+        assert_raises_rpc_error(
+            -8, "Mocktime can not be negative: -1.",
+            self.nodes[0].setmocktime, -1)
+        assert_raises_rpc_error(
+            -8, "Mocktime can not be negative: -9223372036854775808.",
+            self.nodes[0].setmocktime, -9223372036854775808)
+
     def _test_uptime(self):
         wait_time = 10
         self.nodes[0].setmocktime(int(time.time() + wait_time))