No, you cannot change the time and date stamp of transactions manually.
Imonggo always records the transaction in real-time based on the server's absolute time (also called UTC or GMT time). To display the correct local transaction time, you may have to set the timezone manually.
To change the timezone, go to Office > Control Center, and click Settings under Manage Account.
If you are using Store v2, Imonggo will use your PC's current time setting for the invoice time.