"Coming Soon" pages and domain age

I am in the process of building a website and will be ready to launch in 30 days. I already have my domain and hosting. I would like my site to get in and out of the Google sandbox as quickly as possible (naturally).

If I put up a substantial placeholder page (with a good volume of actual text about my upcoming site, not just a silly “under construction” message) and submit it to search engines, will they calculate my domain age from that point after my full site launches later?

Google will calculate your domain age from the time of registration as in whos detail, it doesn’t matter there is content or under construction.

You no longer need to worry about the Google sandbox, these days its non-existent. I registered a domain a month ago and had it sitting on a parked page. I launched a site on it last week and it was indexed in Google within days.

Google is automated they don’t manually check your site to see if you put something like under construction or not.

the bot will crawl your site, check what keywords you have in Title, description and body text and how many links are there to pages, and what anchor they use

where will you rank in SERPS depend only on the above factors, the so called sand box, is a natural result of high competition on some keywords, It doesn’t latterly exist