Using PHP ...
This is for my personal use so I'm thinking maybe 3-4 emails a day. I'm at a point where I can send an email to a dedicated email address where my script parses the message and stores it into a DB. Now, I need to figure out the best way to check the records in the DB for any upcoming task. I feel like I'm missing something, maybe like a trigger field as to when a reminder should go out. However, that's not a concern to me at the moment since I'll just send an alert 15 mins prior to the due date.
Question is, shoudl I run a cron job that queries the DB every minute? I take it the query will have to say something like "select all tasks that is due within 15 minutes."
A cron job seems reasonable. You just query based on the dueDate:
MySQL: SELECT * from jobs WHERE dueDate > NOW() + INTERVAL 15 MINUTES
SQL Server: SELECT * from jobs WHERE dueDate > DATEADD(GETDATE(),15,minute)
A quick cron job should be fine, for something small. It had better be a quick query though, or you'll get booted if it's a shared server.
Why don't you use those free cron services if on shared server? For something this simple, that is your best bet!