Scholarly opinion is as follows:
The author or composition date of Mudgala Upanishad is unknown. Jan Gonda – a professor of Sanskrit and Dutch Indologist, states it is a late Upanishad. The style and structure of Sanskrit words used by the text suggest it to be a medieval text. The text, states Klaus Witz, is a post-Vedic but early Vaishnava Upanishad. The Upanishad reflects an integration and a harmony of Vedic doctrines with those of Vaishnava doctrines.
The Subala Upanishad, together with the relatively older Mudgala Upanishad, are two Upanishads that discuss the Purusha Sukta of Rigveda, both notable for asserting that Narayana (Vishnu) is the Brahman (Highest Reality, Supreme Being). The Subala Upanishad text differs from Mudgala Upanishad in presenting more verses of the Purusha Sukta, being longer, and for declaring Narayana to be the father, the mother, the refuge, the friend and the goal of every living being.
The author or composition date of Subala Upanishad is unknown. Hans Hock states that it is a late Upanishadic text.
Is this the consensus as to the dates of these texts?