Convert your dates to the format you want before converting to JSON by using a calculated property. For example:
Get-ItemProperty -Path .\AbraSuite01.03.2014 | select Name,@{Name="CreationTime";Expression={$_.CreationTime.ToShortDateString()}} | ConvertTo-JSON
Replace $_.CreationTime.ToShortDateString()
with whatever you need to get the date format you're after - but the key here is to use $_.CreationTime
to grab the creation time of the object in the pipeline.