The PhotoUp system is set up so that at 12:00 am EST on the 1st of the month is when your account will switch from one month to the next. This means that all images that finish uploading after 12:00 am EST on the 1st of the month will be credited to the new month.
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As long as images are finished uploading to the system before 12:00 am EST on the last day of the month, those credits will be deducted from the current month.