If you don't know the exact time of birth, put in the nearest time that you know. At least it will generate a birth chart to which you can refer as you read along. If you just know the birth date but do not know the birth time at all, one of the options will be to put in 12.00 noon of your birth date in order to produce a birth chart just for the purpose of study or for the aim of setting a rough estimate.
After you are well-versed in Bazi analysis, even not having the time of birth will be able to tell a lot of information about someone's fate, inherent traits, personality and destiny. Nonetheless, knowing the time of birth, to the minutes, will left nothing to chance and provide a very accurate analysis.