Previous month fails in February/March

[neufeind]

In app/models/periodictask.rb the values for PREVIOUS_MONTHNAME and PREVIOUS_MONTH are calculated as "now - 2592000", where 2592000 is 30 days (in seconds). Obviously this fails at least once a year at the beginning of March, because February has 28 or 29 days.
On 1st March
expected: February
actual result: January

Maybe subtract the current day of month (multiplied by 86400) to return the last day of the previous month correctly?