Following is just a sample to get the current site’s last modified date in UTC. You can apply the same to any DateTime fields in SharePoint. Here, SharePoint itself returns the DateTime in UTC as per its regional setting.

ClientContext ctx = new ClientContext(weburl);
ctx.Credentials = new SharePointOnlineCredentials(userName, passWord); 
RegionalSettings regionalSettings = ctx.Web.RegionalSettings;

ClientResult<DateTime> utcTime = regionalSettings.TimeZone.LocalTimeToUTC(ctx.Web.LastItemModifiedDate);

ctx.ExecuteQuery();

utcTime.Value is your DateTime in UTC.