Format Functions in SSRS Expressions

User-Defined Numeric Formats (Format Function) - 

http://msdn.microsoft.com/en-us/library/4fb56f4y%28v=VS.80%29.aspx

User-Defined Date/Time Formats (Format Function) -

 http://msdn.microsoft.com/en-us/library/73ctwf33%28v=VS.80%29.aspx 

Format Function - http://msdn.microsoft.com/en-us/library/59bz1f0h%28v=VS.80%29.aspx

Example:

Date Format in SSRS
=format(Fields!IssueDateTime.Value,"yyyy-MM-dd HH:mm:ss")