Can you please look at this:

trigger AccountSimpleTrigger on Account (before update) {

    ////////////////////////////////////
    // This trigger detects the change of the TestCheckbox field
    // and creates an Event
    ////////////////////////////////////
    
    if(trigger.isUpdate && trigger.isBefore) {
        
        // Create empty List to hold new Events (bulk DML) 
        List<Event> lstNewEvents = new List<Event>();
        
        for(Account a : trigger.new) {
            
            // Determine if the checkbox is TRUE and the user has just flipped it from FALSE.
            if(a.TestCheckbox__c == true && a.TestCheckbox__c != trigger.oldMap.get(a.Id).TestCheckbox__c) {
                
                // Set Start Date
                Datetime dtmStart = a.TestDate__c;
                Datetime dtmEnd = a.TestDate__c;
                Long lngClassDuration = (dtmEnd.getTime() - dtmStart.getTime()) / 60000;
                
                // Create Event
                Event newEvent = new Event();
                newEvent.OwnerId = UserInfo.getUserId(); // Sets the current userId as the Event Owner
                newEvent.WhatId = a.Id; // Sets the Account as the Event's "Related To"
                newEvent.Subject = 'New Event - Some Reason';
                newEvent.IsAllDayEvent = true;
                
                
                lstNewEvents.add(newEvent);
                
            }
        }
        
        if(lstNewEvents.size() > 0) { insert lstNewEvents; }

I have a mistake:

Error:Apex trigger AccountSimpleTrigger caused an unexpected exception, contact your administrator: AccountSimpleTrigger: execution of BeforeUpdate caused by: System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [Date]: [Date]: Trigger.AccountSimpleTrigger: line 36, column 39

I am so close -I know you regret answering my first post, but if you knew how much I appreciate this :-)
        

hi this is the error:

Error: Compile Error: expecting right curly bracket, found 'EOF' at line 0 column -1

trigger ErnestMoney on rethink2__Closing__c (before update) {

    ////////////////////////////////////
    // This trigger detects the change of the TestCheckbox field
    // and creates an Event
    ////////////////////////////////////
   
    if(trigger.isUpdate && trigger.isBefore) {
       
        // Create empty List to hold new Events (bulk DML)
        List<Event> lstNewEvents = new List<Event>();
       
        for(rethink2__Closing__c a : trigger.new) {
           
            // Determine if the checkbox is TRUE and the user has just flipped it from FALSE.
            if(a.TestCheckbox__c == true && a.TestCheckbox__c != trigger.oldMap.get(a.Id).TestCheckbox__c) {
               
                // Set Start Date
                Date dteStart = a.TestDate__c;
               
                // Create Event
                Event newEvent = new Event();
                newEvent.OwnerId = UserInfo.getUserId(); // Sets the current userId as the Event Owner
                newEvent.WhatId = a.Id; // Sets the Account as the Event's "Related To"
                newEvent.Subject = 'New Event - Some Reason';
                newEvent.ActivityDate = a.TestDate__c;
                newEvent.IsAllDayEvent = true;
               
                lstNewEvents.add(newEvent);
               
            }
        }
       
        if(lstNewEvents.size() > 0) { insert lstNewEvents; }
       
    }