Update: The answer previously posted here linked to a custom script that is no longer available, solely because the OP indicated that date +'%s' didn't work for him. Please see UberAlex' answer and cadrian's answer for proper solutions. In short:
For the number of seconds since the Unix epoch use date(1) as follows:
date +'%s'
For the number of days since the Unix epoch divide the result by the number of seconds in a day (mind the double parentheses!):