Please help me.. im in a very tricky situation... i've been searching for job lately and finally i got one ...

i'm suppose to start shortly ... and just a couple of days later i have another interview which is for a much better job but its during day time ... what should i do ?.... what excuse shud i make in the very first week of my job or shud i go for an interview reschedule ... to sometime after 5 pm ... or on a saturday .... wud they entertain my request?