I'm not even convinced there has to be a critical period when mothers "have" to be full time. Nobody says that children will ruined forever if their fathers aren't full time fathers for six weeks, let alone six years. What's important is that children of every age have reliable adults who care for them, provide consistent structure, and love them. The more the better. I am a great believer in lots of adults in a child's life. They can be relatives, friends or hired help. They key is that they be the right person.
When my kids were little, I used both baby sitters and various kinds of day care and there was never any question of my kids not loving me! They are 16 and 20, don't do drugs or have any major issues, and are close to me and affectionate.
I read one study that showed that relatives provided the lowest level of care to young children--were the most likely to neglect and leave them in front of the television for long periods, compared to a professional babysitter or day care provider.