This paper shows that the flexibility of time as a potential of Computer Assisted Language Learning (CALL) proves the sustainability of its latest stage -- Ubiquitous CALL. Firstly, the ideas of the Third Millennium Pedagogy are combined with the ethical and philosophical principles of sustainable education. One of the descriptors of the sustainable education -- its durability is then compared with the principle of time flexibility in e-learning. On this theoretical base, the ideas of Ubiquitous Learning in general and Ubiquitous CALL (U-CALL) in particular are developed. Two comparable cases of the research aimed at the time flexibility of e-learning in foreign language education are described. The former one was conducted in 2016, the later in 2008. Similar results in time flexibility prove that e-learning in language education can be omnipresent, fulfilling the demands on Ubiquitous CALL. The durability of time flexibility proven by the real-life examples indicates the sustainability of U-CALL.