Changing character into date

Dear R People:

I have a column in a data frame that has date and time (one per
minute), such as "2012-08-09 22:23:00 2012-08-09 22:24:00" etc.

What is the best way to change that into numeric dates to be used as
an x-axis, please?

I've been looking at as.Date and as.Date.numeric, but those are for
monthly/yearly data.

Thanks for any help in advance!
Sincerely,
Erin

Erin Hodgess
Associate Professor
Department of Computer and Mathematical Sciences
University of Houston - Downtown
mailto: erinm.hodgess at gmail.com