MinGW As is: ============ Qt 5.12.5 (x86_64-little_endian-llp64 shared (dynamic) debug build; by GCC 8.1.0) SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." SetTimer 4294967294 returns -2 KillTimer -2 fail "The operation completed successfully." MinGW With fix: =============== Qt 5.12.5 (x86_64-little_endian-llp64 shared (dynamic) debug build; by GCC 8.1.0) SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok MSVC as is: =========== Qt 5.13.1 (x86_64-little_endian-llp64 shared (dynamic) debug build; by MSVC 2019) SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok SetTimer -2 returns -2 KillTimer -2 ok MSVC With fix: =============== c:\dev\5-vs17-513\qt-513\qtbase\src\corelib>..\..\examples\widgets\mainwindows\mdi\debug\mdi.exe Qt 5.13.1 (x86_64-little_endian-llp64 shared (dynamic) debug build; by MSVC 2019) SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614 KillTimer 18446744073709551614 ok SetTimer 18446744073709551614 returns 18446744073709551614