I believe the 5th and 6th parameters to xp_ReadErrorLog are start and stop datetimes respectively, so you should be able to do something like:
EXEC xp_ReadErrorLog 0, 1, Null, Null, '2012-05-16 00:00:00', '2012-05-17 23:59:59'
Here is my understanding of the parameters:
Parameter 1 = log version (0 = current)
Parameter 2 = log type (1 = error, 2 = agent)
Parameter 3 = Search string
Parameter 4 = Search string
Parameter 5 and 6 I believe are start/end datetime restrictions
Hope this helps.