The Proto-Indo-European word *sem-
Word *sem-
Meaning at one time, once
See also Proto-Indo-European: *semgo(lo)s ‘single one’
Proto-Indo-European: *sem- ‘to put in order/together’
Proto-Indo-European: *som- ‘(together) with’
Proto-Indo-European: *somos ‘same’
Proto-Indo-European: *sm̥mós ‘some, any’
Proto-Indo-European: *sems ‘united as one, one together’
Proto-Indo-European: *sm̥teros ‘one or the other of two’

Comments
Mallory J.P. and Adams D. (2006)

Other words that might be of interest
*wet- ‘year’, *h₁en- ‘year’, *kēs(k̂)ehₐ ‘time’, *kʷóm ‘when’, *kʷodéhₐ ‘when’, *(h₁)yēro/ehₐ ‘year, new season’, *hₐetnos ‘year’, *posti ‘after’, *wételos ‘yearling’