In September I added 26 calendar events, one every other Saturday, to set personal goals. Each calendar entry is a separate event, I am not using the Recurring Event feature. I've noticed that some goals are shifting a few days into the future.
The events from 29 November to 13 December have shifted two days forward: instead of being on a Saturday they appear on the Monday two days later.
The events from 27 December to 10 January have shifted five days forward: instead of being on a Saturday they appear on the Thursday five days later.
The events from 24 January to 7 March have shifted eight days forward: instead of being on a Saturday they appear on the Sunday eight days later.
The events from 21 March to 4 April have shifted eleven days forward: instead of being on a Saturday they appear on the Wednesday eleven days later.
And so on, the gaps get larger further into the future.
The only other event that I have added so far out is a recurring event every Tuesday, which is not affected (i.e. has not shifted). I do not cross time zones with this device, and the day shifting does not correlate with any Day Light Savings Time shifts. This is a work phone with a lock screen: my children nor anybody else has access to the device nor to the Google account with which it is associated to. I have very few apps installed on the device, such as Anki and Firefox. I do not have any social media apps, such as Facebook, on the device.
This is on a Samsung Note 3 on Android 4.4.
More information: This is using the stock Google calendar application, with the calendar stored and synced with my Google account 'dotancohen'. As these are the only entries in the 'dotancohen' calendar that far in advance, I do not know if the other entries would have be affected. I do have other calendars which are shown in the Google calendar application, connected to other Google accounts (such as my work account) are not affected. All the affected entries we added one at a time by hand on the device, i.e. I long-clicked the calendar in Month mode and manually added each entry. For that particular calendar (the 'dotancohen' Google account) I have never opened the calendar on another device.