If your social media posts are not posting at the exact time set on your publishing schedule, but are within 10 minutes of the scheduled time (before or after), it's likely that you have the Vary publishing times setting enabled.
To see if this setting is turned on, navigate to Social > Publishing and click Publishing Schedule in the left sidebar. The Vary publishing times checkbox appears beneath the chart of your scheduled publishing times. If you'd like to disable this setting, click to uncheck the box.
If your posts are not being published successfully, check out this article for more information about the most common unsuccessful social message errors.