Make a new theme with a file called Info.plist that contains the following (enable in Winterboard):
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>TimeStyle</key>
<string>display:none</string>
</dict>
</plist>
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>TimeStyle</key>
<string>visibility:hidden</string>
</dict>
</plist>
Once of these may work. Granted I cannot test it because I just re-jailed my phone, but the properties are essentially CSS.
