DOL (Department of Labor) SIMPLE IRA Plans for small business publication:
Most of the information is the same as the IRS, however we noticed this more stringent salary deferral deposit requirement:
For plans with fewer than 100 participants employers can deposit salary eduction contributions with the plan no later than the 7th business day after withholding it to be considered in compliance with the law.
The DOL is the same as described in the IRS publications for employer contributions:
You must make your employer contributions by the due date for filing your business’s Federal income tax return for the year (including extensions, if applicable).