formula issue

[samuel_brierley]

Hi I have the follwing formula, admittedly the community helped me with it,

however I need it to do the same thing but instead of being before today it needs to be before the first day of the month?

sum({$< Date = {'>$(=Today())'}>} [Forecast Hours]) 

[Sokkorn]

sum({$< Date = {'>$(=MonthStart(Today()))'}>} [Forecast Hours])