One day late, for a good reason - moving to another job that will enable continued payment of rent - would not concern me, if informed in a way that acknowledged the tenant knew late payment wasn't ok.
The phrase 'what can you do? ' (used by pp) strikes fear into my heart.
It's indicative of a passive, irresponsble attitude to life, whereby you do whatever you consider to be your best, hope for the best and expect everyone around you to accept that's enough, mop up your problems and pay the difference.
Tenancies are based on contracts and trust, not on a sense of goodwill and 'doing ones best'. If you can't meet your contractual obligations, then, after first exploring every possible avenue to cover short-term problems - overdraft, loans, delay less pressing payments, sell things, do extra work etc, you have to change your circumstances - find cheaper accommodation, or a better paying, or more reliable job.
'What can I do? ' quickly accompanies rent being a week late, a month, or not paid at all, as and when the tenant's work or personal circumstances change.
I suspect that's where the landlord is coming from. You need to reassure him that you know this isn't ok, you don't see him as a bank offering free loans, you are moving into a reliable, salaried job and you will always prioritise paying rent over other expenses.