Replacement for Oracle's "ROWNUM" in Postgres

This query is setting a date into a column for all the rows, starting with a
fixed date with 10 minutes intervals.

UPDATE some_table
SET some_date_column = 'some_literal_date' + (ROWNUM / 144.0);

It is used to initialize a table with data.
Is there any way to get the same result in Postgres somehow ?
Of course there is always the possibility to do it in a loop and calculate
the dates in the program, but that sucks.

Thanks,
Csaba.