As per answer by @ProVi just change to suit the formatting you require
echo %DATE:~10,4%-%DATE:~7,2%-%DATE:~4,2% %TIME:~0,2%:%TIME:~3,2%:%TIME:~6,2%
will return
yyyy-MM-dd hh:mm:ss
2015-09-15 18:36:11
EDIT As per @Jeb comment, whom is correct the above time format will only work if your DATE /T command returns
ddd dd/mm/yyyy
Thu 17/09/2015
It is easy to edit to suit your locale however, by using the indexing of each character in the string returned by the relevant %DATE% environment variable you can extract the parts of the string you need.
eg. Using %DATE~10,4% would expand the DATE environment variable, and then use only the 4 characters that begin at the 11th (offset 10) character of the expanded result
For example if using US styled dates then the following applies
ddd mm/dd/yyyy
Thu 09/17/2015
echo %DATE:~10,4%-%DATE:~4,2%-%DATE:~7,2% %TIME:~0,2%:%TIME:~3,2%:%TIME:~6,2%
2015-09-17 18:36:11