If you would like to have a different greeting for after office hours, here is the guide for it.
Setup a Service Flag
You will first need to setup a Service Flag. Below is an example where the office hours are between 9am and 5pm, Monday’s through Friday’s and is closed during the weekend:

Setup an Auto Attendant
When setting up your Auto Attendant, in the Upload Custom Greeting field, you will want to upload a greeting to greet your callers when your office is open.
Once the file has been uploaded, scroll down to the Service Flag section and in the first available Service Flag, select the Service Flag that you created in the previous step and upload a different greeting that will greet your callers when your office is closed.
You will also need to setup your Timeout Management as this will instruct the Auto Attendant what to do after the greeting has been played. In this scenario, for the Number of times to repeat welcome message, you can enter the value 2. In the Redirect Number, you can redirect it to a voicemail only extension ( )
This would mean that the Auto Attendant will play the greeting twice and then automatically forward the call to a voicemail extension.
Number of times to repeat welcome message is active for ALL you greetings, which covers the main greetings or any other greeting within your Service Flags.