Hi I have date which is in text format, sample date is "2013-04-02 00:00:00

Did you try date()
forum.appian.com/.../Date_and_Time_Functions

For whatever reason, the ISO 8601 date format (YYYY-MM-DD) is not supported by the todate() function (or by the date picker); in your case you can try splitting the date and time apart using the split() function or similar, then following the previous commentor's suggestion, call the date() function after splitting the date portion on the dash ("-") character.

I just wrote this example solution which utilizes the with() function to help handle the splitting. You can leave out the last little bit if you don't need the passed-in time at all.
with(
dt:split("2013-04-02 00:00:00.000"," "),
d:split(dt[1],"-"),
date(d[1],d[2],d[3])&" "&totime(dt[2])
)

thanks to all