MIL pissed at us

Ok so we are going on a date for our anniversary to eat and have movie tickets for 7pm already bought and we told mil we were dropping him off at 4ish and at 4:45 his mom still wasn't home so we just went and left him at my moms and now his mom is pissed at us because she was “5m away”. Wtf. Should we have waited longer? She’s notorious for saying she’s almost there when she is not.