I see no reason for a date table from what you describe. By "linking" you've likely created as situation where you're trying to add a duplicate date into the PK field of the date table, which again, doesn't seem to be required. Nor do I think the form/subform design is correct, unless you want to see all maintenance records regardless of what piece of equipment has such records. If not, then the design is suspect.
What you probably need is all desired equipment info on the main form and a subform that also contains a hidden field for the equipment id. The form/subform child relationship is set to be the id field. That will provide all maintenance records for an equipment id, to which you can add new records if desired. This will also expose existing records to edits or deletions, so a separate input form for these records might be a better method.
If none of that helps, a pic of your table relationships might help and provide more information for evaluation.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.