The Daily Rewards Calendar is there to give players a steady stream of rewards that increase in value if you continue to log in every day for fifteen days.
Collecting the first reward starts the fifteen day timer; if you collect the first reward on the 1st day of the month, then the Daily Rewards Calendar will be resetting on the 16th day of the month at 12:00 PM UTC. This means that if you log in every day for fifteen days, you'll collect all 15 rewards (up to and including 250 Gems!), but any days missed will result in the later rewards being unavailable.
For example, if you log in and collect the first reward on the 1st day of the month, you will receive an Experience Lockbox. If you take a break from the game after that, and do not log back in until the 10th day of the month and log in every day after that, then you will only be eligible to collect the next five rewards (resulting in six rewards in total collected) before the Daily Rewards Calendar resets.
Please keep in mind that each new day's reward from the Daily Rewards Calendar is available at 12:00 PM UTC, whether or not you collected the prior day's reward at 12:01 PM UTC or 11:59 UTC. Depending on your timezone and its relation to the UTC timezone, it is possible to log in on consecutive days from your perspective, but miss a day by UTC.
For example, 12:00 PM UTC is 5:00 AM Pacific Daylight Time. If I am on Day 5 and collect Day 5's rewards at 1:00 AM Pacific Daylight Time on the 5th day of the month, Day 6's rewards will be available four hours later at 12:00 PM UTC / 5:00 AM PDT on the 5th day of the month. If I do not collect my Day 6 rewards until 9:00 AM PDT on the 6th day of the month, I will have lost a day for the Daily Rewards Calendar and will not be able to collect the Day 15 reward, even though it still felt like I logged in on two consecutive days from my perspective, because it was more than 24 hours since my last collection and the Daily Rewards Calendar will have counted down another day since then.
If you want to collect all 15 rewards, then you will need to log in consecutively for 15 days based on the UTC timezone. Please note that Rumble Player Support is not able to provide replacements for rewards missed due to non-consecutive logins.