Numerals have 6 case forms, like most nouns. They're described in detail in Numerals: Case Forms. The only case you need for telling time is the Genitive. Numerals ending in -ь belong to the ь-declension and have the normal Genitive ending -и (без десятќ). The numeral одќн is a special adjective and is feminine in this time expression in order to agree with the Genitive of the word минџта 'minute': без однћй минџты вћсемь '7:59'. The numerals 2, 3, and 4 have these Genitive endings:
двђ, двљдвџх
In the following examples the Genitive forms governed by без are in italics.
Ї встђла в двљ минџты шестћго. I got up at 2 minutes after 5.
Ї встђла без двџх минџт пїть. I got up at 2 minutes of 5.
Ї встђла в двђдцать пїть минџт шестћго. I got up at 25 minutes after 5.
Ї встђла без двадцатќ пятќ пїть. I got up at 25 minutes of 5.
Recall the rule: after the forms двђ, двљ, трќ, четІре the accompanying noun is in the Genitive Singular. This rule applies to минџты in the first example above (двљ минџты). However, it does not apply to the second example (без двџх минџт), where the form минџт is the Genitive Plural of минџта.
The reason the rule does not apply is that the rule specifies the particular forms двђ, двљ, трќ, четІре (Accusative inanimate and Nominative), not their inflected forms (like Genitive двџх).
In the phrase без двџх минџт the word минџт is Genitive because it is governed by без and it is Plural because the meaning is plural ('more than one minute').