Set Transient expiration

I have a page that displays a random post every day. In order to change post every 24 hours I use

set_transient('totd_trans_post_id', $totd[0]->ID, (60*60*24));

What I’m wondering is this:

• Does the countdown (60*60*24) start as soon as I have inserted and saved the code?
• If yes: What happens if I let the code run for 23 hours, and then decide to “update” the code. Will the countdown restart?
• If no one visits the site for 48 hours, will the expiration still take place? Or does someone have to visit after 24 hours to “run” it?

1. The countdown would start as soon as the transient is created or updated.

2. Running set_transient() on an existing transient value will restart the clock. Per the Codex page on set_transient():

   If a transient exists, this function will update the transient’s expiration time.

3. According to the Transients API page, the expiration time is the maximum lifetime of a transient value. It may be deleted before the time is up, but it will never return its value after the time is up:

   Transients may expire before the $expiration (Due to External Object Caches, or database upgrades) but will never return their value past $expiration.