This AppleScript will lock your Mac at 5pm and midnight [see installation info below]

com.tjluoma.lockscreen.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>Disabled</key>
	<false/>
	<key>Label</key>
	<string>com.tjluoma.lockscreen</string>
	<key>ProgramArguments</key>
	<array>
		<string>/usr/bin/osascript</string>
		<string>-e</string>
		<string>tell application "System Events"</string>
		<string>-e</string>
		<string>keystroke "q" using {control down, command down}</string>
		<string>-e</string>
		<string>end tell</string>
	</array>
	<key>RunAtLoad</key>
	<false/>
	<key>StandardErrorPath</key>
	<string>/tmp/com.tjluoma.lockscreen.log</string>
	<key>StandardOutPath</key>
	<string>/tmp/com.tjluoma.lockscreen.log</string>
	<key>StartCalendarInterval</key>
	<array>
		<dict>
			<key>Hour</key>
			<integer>0</integer>
			<key>Minute</key>
			<integer>0</integer>
		</dict>
		<dict>
			<key>Hour</key>
			<integer>17</integer>
			<key>Minute</key>
			<integer>0</integer>
		</dict>
	</array>
</dict>
</plist>

1. (Optional) Edit the plist to change the times when your Mac will be locked. "Hour 0" is midnight. "Hour 17" is 5 p.m. You can adjust those at will.¹

2. Save this to ~/Library/LaunchAgents/com.tjluoma.lockscreen.plist

3. Add /usr/bin/osascript to System Preferences » Security & Privacy » Privacy » Accessibility (click on the + and then nagivate to /usr/bin/osascript so it appears in the list. Make sure it is ✔️ checked.

¹ Use LaunchControl to avoid having to edit launchd plists by hand.

• Note that this will only work on High Sierra and later as that is when ⌘⌃Q was added as global Lock Screen command