First day of current month:
{DATEFORMAT(M/1/yyyy)}
Last day of current month (get first day of current month, add a month, then subtract a day):
{DATEADD(Days|{DATEADD(Months|{DATEFORMAT(M/1/yyyy)}|M/d/yyyy|1|M/d/yyyy)}|M/d/yyyy|-1|M/d/yyyy)}
You can make these user-defined variables so you only have to set these once.