$dateTimeStr = '20141231T23:59:59'
$format = 'yyyyMMddTHH:mm:ss'
$formatProvider = [Globalization.CultureInfo]::InvariantCulture
[datetime]::ParseExact($dateTimeStr,$format,$formatProvider)
 
#2014年12月30日 23:59:59

PS> $now=Get-Date
PS> $d.ToString('yyyyMMddTHH:mm:ss')
20141231T00:22:24