How can I display my custom metaboxes on a custom post template?

Looking at your code, it appears all meta keys are prefixed with _cmb_. Your start time key is text_datetime_timestamp, so combined with the prefix, you want to fetch:

get_post_meta( get_the_ID(), '_cmb_text_datetime_timestamp', true );

Note the third parameter, which will retrieve a single value if set to true, as it’s possible to save multiple values under a single key, which would result in an array of values.

The simplest way to see all of the keys your meta data is saved under, is via get_post_custom_keys:

$custom_field_keys = get_post_custom_keys();
foreach( $custom_field_keys as $key => $value ) {
    echo $key . " => " . $value . "<br />";
}

Put that in the loop and it will tell you the names of all of the keys your meta data is saved under.