Rather, רבא said; according to ר"מ a person errs slightly less than two hours - אלא אמר רבא לדברי רבי מאיר אדם טועה ב' שעות חסר משהו
When two witnesses testify different times we assume that one is exact and the other errs; we do not assume that they are both in error. We do not attempt to reconcile the apparent contradictory testimony of עדים (by scrutinizing the witnesses) to be stringent with the defendant.
There are two (alternate) purposes for asking ‘which day’. If the עדים contradict each other [or themselves] concerning [the date and] the day, their testimony is disqualified. Alternately if the dates of the month are off by one day the עדות can be upheld if they agree on the day of the week.