The dates on the packages is when their final commit was made for Bootstrap 4. I'm not sure why you're manually comparing dates though and should always compare version number if done manually. The version checking lets you know of updates available and the version cache refreshes every 24 hours. You can use the Install from Web built into CB within CB > Plugin Management > Install New Plugin as well to check for updates as it specifically has an Updates section. That cache is also for 24 hours, but you can force it to refresh by clicking the refresh link.