Once you know the name of an object type, you can use that type for conversion. The next line converts a string into a date-time type:
[DateTime] '4.5.2010'
You should note that conversion uses the culture-neutral date format which happens to be the U.S. format. If you need to use your local date format, you should use the Parse() method found inside the DateTime type:
[DateTime]::Parse('4.5.2010')