If the booking was made online, your business will have received a confirmation email of the original booking. Find this email in your email inbox and you will find the original time and date of the accidentally moved booking.
If the booking was made internally, you won't have this confirmation email so you can identify the original date and time of the booking from the confirmation sent to the customer. Go to the customer tab and locate their customer record then go to the communications tab to find all emails and SMS sent to the customer, including the date and time.
If you're finding that you're doing this regularly, you can turn on emails to be sent when a booking is modified so you're asked "are you sure?":
Go to the communications tab
Click emails
Turn on ‘Send customers an email when a booking is requested’.
Now, when you accidentally move a booking, a message will appear asking if you want to send an email to notify the customer of the adjustment. Your options are to click YES/NO or the ‘X’ to cancel in the top right hand corner. Clicking ‘X’ to cancel the move and revert the booking to its original state.