The time you pick is the time your video goes live. Each post publishes to YouTube on your posting days, at your time of day, in the timezone you set. Times run on the hour or the half hour.
What happens before the posting time
Up to two hours ahead: Chillframe starts making the post, so it's ready in time.
Usage: the post uses usage while it's made, not when it publishes. If a post can't finish, the usage it took comes back.
Manual approval: if the Series is set to Manual approval, the finished post waits for you instead of going out. Approve it before the posting time and it goes out at that time. Approve it later and it goes out as soon as it's ready.
When a posting time passes without a post
If the Series can't post at a posting time, that post is skipped and the Series moves on to the next one. Skipped posts are never sent out late, so you never come back to a pile of posts landing on your channel at once.
Change the time or timezone
Open the Series and go to the Schedule tab.
Choose Edit posting window.
Change the Time or the Timezone.
Choose Save changes.
The upcoming posts move to the new time automatically.
Posting days aren't in that window. To post on different days, make a new Series, or ask a connected agent to change the days for you.
The reasons a post gets skipped, and the fix for each, are in Why a scheduled post was skipped.