diff --git a/Core/HLE/sceKernel.cpp b/Core/HLE/sceKernel.cpp
index 38bc5e955..6eb1a310f 100644
--- a/Core/HLE/sceKernel.cpp
+++ b/Core/HLE/sceKernel.cpp
@@ -734,8 +734,8 @@ const HLEFunction ThreadManForUser[] =
 	{0X71BC9871, &WrapI_II<sceKernelChangeThreadPriority>,           "sceKernelChangeThreadPriority",             'i', "ii"      },
 	{0X446D8DE6, &WrapI_CUUIUU<sceKernelCreateThread>,               "sceKernelCreateThread",                     'i', "sxxixx", HLE_NOT_IN_INTERRUPT },
 	{0X9FA03CD3, &WrapI_I<sceKernelDeleteThread>,                    "sceKernelDeleteThread",                     'i', "i"       },
-	{0XBD123D9E, &WrapI_U<sceKernelDelaySysClockThread>,             "sceKernelDelaySysClockThread",              'i', "x",      HLE_NOT_IN_INTERRUPT | HLE_NOT_DISPATCH_SUSPENDED },
-	{0X1181E963, &WrapI_U<sceKernelDelaySysClockThreadCB>,           "sceKernelDelaySysClockThreadCB",            'i', "x",      HLE_NOT_IN_INTERRUPT | HLE_NOT_DISPATCH_SUSPENDED },
+	{0XBD123D9E, &WrapI_U<sceKernelDelaySysClockThread>,             "sceKernelDelaySysClockThread",              'i', "P",      HLE_NOT_IN_INTERRUPT | HLE_NOT_DISPATCH_SUSPENDED },
+	{0X1181E963, &WrapI_U<sceKernelDelaySysClockThreadCB>,           "sceKernelDelaySysClockThreadCB",            'i', "P",      HLE_NOT_IN_INTERRUPT | HLE_NOT_DISPATCH_SUSPENDED },
 	{0XCEADEB47, &WrapI_U<sceKernelDelayThread>,                     "sceKernelDelayThread",                      'i', "x",      HLE_NOT_IN_INTERRUPT | HLE_NOT_DISPATCH_SUSPENDED },
 	{0X68DA9E36, &WrapI_U<sceKernelDelayThreadCB>,                   "sceKernelDelayThreadCB",                    'i', "x",      HLE_NOT_IN_INTERRUPT | HLE_NOT_DISPATCH_SUSPENDED },
 	{0XAA73C935, &WrapV_I<sceKernelExitThread>,                      "sceKernelExitThread",                       'v', "i"       },
diff --git a/Core/HLE/sceKernelThread.cpp b/Core/HLE/sceKernelThread.cpp
index 5685edb47..146d69676 100644
--- a/Core/HLE/sceKernelThread.cpp
+++ b/Core/HLE/sceKernelThread.cpp
@@ -2413,6 +2413,17 @@ static s64 __KernelDelayThreadUs(u64 usec) {
 	if (usec < 200) {
 		return 210;
 	}
+
+	if (usec > 0x8000000000000000ULL) {
+		// Wrap around (behavior seen on firmware) and potentially wake up soon.
+		usec -= 0x8000000000000000ULL;
+	}
+	if (usec > 0x0010000000000000ULL) {
+		// This will probably overflow when we convert to cycles.
+		// Note: converting millenia to hundreds of years.  Should be safe, basically perma-delay.
+		usec >>= 12;
+	}
+
 	// It never wakes up right away.  It usually takes at least 15 extra us, but let's be nicer.
 	return usec + 10;
 }
@@ -2439,40 +2450,38 @@ int sceKernelDelayThread(u32 usec) {
 	return hleLogSuccessI(SCEKERNEL, 0, "delaying %lld usecs", delayUs);
 }
 
-int sceKernelDelaySysClockThreadCB(u32 sysclockAddr)
-{
+int sceKernelDelaySysClockThreadCB(u32 sysclockAddr) {
 	auto sysclock = PSPPointer<SceKernelSysClock>::Create(sysclockAddr);
 	if (!sysclock.IsValid()) {
-		ERROR_LOG(SCEKERNEL, "sceKernelDelaySysClockThreadCB(%08x) - bad pointer", sysclockAddr);
-		return -1;
+		// Note: crashes on real firmware.
+		return hleLogError(SCEKERNEL, SCE_KERNEL_ERROR_ILLEGAL_ADDRESS, "bad pointer");
 	}
 
-	// TODO: Which unit?
+	// This is just a u64 of usecs.  All bits are respected, but overflow can happen for very large values.
 	u64 usec = sysclock->lo | ((u64)sysclock->hi << 32);
-	DEBUG_LOG(SCEKERNEL, "sceKernelDelaySysClockThreadCB(%08x (%llu))", sysclockAddr, usec);
 
 	SceUID curThread = __KernelGetCurThread();
-	__KernelScheduleWakeup(curThread, __KernelDelayThreadUs(usec));
+	s64 delayUs = __KernelDelayThreadUs(usec);
+	__KernelScheduleWakeup(curThread, delayUs);
 	__KernelWaitCurThread(WAITTYPE_DELAY, curThread, 0, 0, true, "thread delayed");
-	return 0;
+	return hleLogSuccessI(SCEKERNEL, 0, "delaying %lld usecs", delayUs);
 }
 
-int sceKernelDelaySysClockThread(u32 sysclockAddr)
-{
+int sceKernelDelaySysClockThread(u32 sysclockAddr) {
 	auto sysclock = PSPPointer<SceKernelSysClock>::Create(sysclockAddr);
 	if (!sysclock.IsValid()) {
-		ERROR_LOG(SCEKERNEL, "sceKernelDelaySysClockThread(%08x) - bad pointer", sysclockAddr);
-		return -1;
+		// Note: crashes on real firmware.
+		return hleLogError(SCEKERNEL, SCE_KERNEL_ERROR_ILLEGAL_ADDRESS, "bad pointer");
 	}
 
-	// TODO: Which unit?
+	// This is just a u64 of usecs.  All bits are respected, but overflow can happen for very large values.
 	u64 usec = sysclock->lo | ((u64)sysclock->hi << 32);
-	DEBUG_LOG(SCEKERNEL, "sceKernelDelaySysClockThread(%08x (%llu))", sysclockAddr, usec);
 
 	SceUID curThread = __KernelGetCurThread();
-	__KernelScheduleWakeup(curThread, __KernelDelayThreadUs(usec));
+	s64 delayUs = __KernelDelayThreadUs(usec);
+	__KernelScheduleWakeup(curThread, delayUs);
 	__KernelWaitCurThread(WAITTYPE_DELAY, curThread, 0, 0, false, "thread delayed");
-	return 0;
+	return hleLogSuccessI(SCEKERNEL, 0, "delaying %lld usecs", delayUs);
 }
 
 u32 __KernelGetThreadPrio(SceUID id)