Update your system time to sync with an internet time server
1)Double click on the date&time displayed on the taskbar on the lower right hand corner of your screen
2)Click "Change Date and Time Settings"
3)Click on "Internet Time" tab
4)Check the box "Synchronize with an internet time server"
5)Chose a server (e.g. time.nist.gov) from the drop-down
6)Click "Update now" button
7)Re-launch Mist and let it finish syncing your node with the block-chain data from its peers before trying to send your transaction again
sudo ntpdate -u time.apple.com
I had the same issue, so I tried Parity, which threw an error: "Your clock is not in sync. Detected difference is too big for the protocol to work: -2254ms. Synchronize your clock."
When I did the above, I was able to re-launch Mist, Parity, and the Ethereum Wallet client and my transactions could go through with any of those (after finishing syncing the node with the block-chain data from network peers)