Hello ​@gerarcat ,
I am assuming that you are collecting the meeting’s date and time in Manychat, and that at some point in your current flow, you are storing this information in a custom field of the "date and time" type.

To have Manychat trigger a reminder before the meeting, you will need to create another custom field, also of the "date and time" type, and set its value right after the step where you collect the meeting's date and time.

In this action, you will save the value from the original meeting field into the new field, applying a -12 hour offset.
This way, you will have a dedicated field with the correct time to trigger the reminder.

Once this is set up, you can create a new flow where the trigger is a contact event based on this newly created "date and time" field.

Make sure you use a channel that allows you to send messages at any time, such as WhatsApp. On Instagram, for example, this will not be possible.

Please let me know if this helped you!

yes man.. thats just what i guess i should do.. but dont see where to set a calculated field… is it where to do it in manychat?

See the screenshot below to locate the action block for saving the time that will trigger a notification 12 hours in advance:  
 

1. This is the field where you want to save the new time, which will be used as the notification trigger.  
2. Here, select the appropriate field type to enter the value from a custom field.  
3. This is your current field, where the meeting date and time are being collected.  
4. Enable this option to define how many hours or days before (or after) the original time you want to save in the new field.

Thnsk a lot bro!… appreciate it… 

and also found a way to set it up in the smart delay, with the dinamic delay taking the appointment date and checking compensation.. there i was able to play with the hours before…