I found a way.
- First day: set a Date(year, month, day) with month and year dropdown's value. The "1" hardcoded represent the day.
Date(yearRangeDate.Selected.Value, monthRangeDate.Selected.Value, 1)
Last day: Copy the first day formula, and sum 1 month. After that subtract 1 day
DateAdd(DateAdd(Date(yearRangeDate.Selected.Value, monthRangeDate.Selected.Value, 1), 1, TimeUnit.Months), -1, TimeUnit.Days)
Full example of a filter:
Filter(
listName,
Date >= Date(yearRangeDate.Selected.Value, monthRangeDate.Selected.Value, 1),
Date <= DateAdd(DateAdd(Date(yearRangeDate.Selected.Value, monthRangeDate.Selected.Value, 1), 1, TimeUnit.Months), -1, TimeUnit.Days)
)