Rule 506.7e

If a spell states that it may be cast “only before [a particular point in the combat phase],” but the stated point doesn’t exist within the relevant combat phase because the declare blockers step and the combat damage step are skipped (see rule 508.8), then the spell may be cast only before the declare attackers step ends. If the stated point doesn’t exist because the relevant combat phase has been skipped, then the spell may be cast only before the precombat main phase ends.

Find this number in the current Rules