my 2007 silverado is hard to start with this cold weather. I had the battry and alternator checked both are good. any ideas on what to check next. My clock resets to zero each night and the turn over of the starter is slow but starts . any ideas out there

 

I would look for a bad connection at the battery or the other ends of the battery cables.
From corrosion or loose.
Then I would look for broken copper in those cables.
Not likely for that age.
Side terminal battery connections?
Remove cables, clean the terminals and cable ends, smear grease on terminals and reinstall. Try to re-install with the cable contact nubbins hitting a new spot on the battery terminals.

The clock reset tells me your loosing all contact with the battery.
But when you crank it over, an arc somewhere welds the lost connection good enough to turn the engine over.
But then vibrations and/or the cold allows contact to be lost again.