Drawing of the screen stops when set the time backward on vncserver.

Issue

In default desktop environments (twm) on vncserver, drawing of the screen stops when time is set by the date command.

How to reproduce:

1. Start vncserver service without editing user's ~/.vnc/xstartup script as a default.
2. Login to this desktop using vnc client.
3. Set to the past time rather than the present time on xterm.
4. Run command, such as top, and check that screen drawing on xterm is not performed.

It reproduces by all means.