There were a lot of customers using TwitteyBot, but only a couple are using the CSV upload facility to schedule the tweets. Both of them were are in the same timezone as I am in, and everything seemed to be in order. However, a couple of weeks ago, a user notified my of an error with the dates in CSV.
There were two errors. The first one was regarding the time format. I had erroneously set the time format of the Java Time formatter to the 12 hr format. Hence, 12 noon was interpreted as 00:00 hrs. All it required was changing the format from hh:mm to HH:MM, as in line 57 of this diff.
The second error was about using the timezones of the browser, when uploading the file. The application was picking up the timezone of the user from the browser and applying it before saving the dates. I was correcting the time, but the date was not corrected. Hence, for anyone to the west of GMT, the time would be corrected, but the date be for the previous day. This is also corrected in lines 282-289.
With these errors corrected, all I have also contacted who have their own instances of the application. If you have cloned the application and are hosting your own version, please do update the application to have this bug fixed, as per this commit.