``Begin'' appears to be more straight-forward (+E8) but suffers from conflicting meanings since it is a common programming language keyword (-E5).
``Start,'' ``commencement,'' and ``origin'' are awkward to use, e.g., ``Start precedes the event,'' ``Commencement precedes the event,'' and ``Origin precedes the event.'' (-E8). Furthermore, choosing start would require us to choose ``end'' for the opposite concept, and end is a common programming language keyword (-E5). Origin also has a conflicting meaning relative to calendars (-E5).
Lastly, ``negative infinity'' is longer (-E2) and slightly misleading since it implies that time is infinite (-E9). This may or may not be true depending on theories about the creation of the universe. Also, negative infinity has a well-established mathematical meaning (-E5).