The are many date fields available in Flex. The first two worth mentioning are the Start Date and End Date. These are the most important because are used for calculating availability.
Start Date and End Date and be captioned and re-captioned in several places throughout the system, so it's important to be aware of those locations and what your company has decided to call them.
To begin, these date options are turned on and off for a given Element in Projects-->Project Elements. For Quotes, you would select Quote, then Dates & Times. Start Date Caption and End Date Caption are the first places to caption these dates. By default, they are "Ship Date" and "Return Date" for Quotes, Pull Sheets*, and Manifests.
For Financial Document type Elements (i.e., Quotes, Invoices, Rental POs, etc.), these dates can be "re-captioned" in each Document View. Refer to Best Practices - Document Views webinar for details.
Also, keep in mind that Report templates do not always honor these captions, so they may be further "re-captioned" in Reports.
Other date fields can be enabled, but not captioned, on the Field Settings tab of the Element's Settings page (Projects-->Project Elements).
*In some systems, the End Date is captioned "End Date" for Pull Sheets.
Dates as they affect Conflicts and Availability
There's a hierarchy to the Start and End Dates (i.e. Ship and Return). The top-most level is the Element header (i.e. the Quote, Pull Sheet, Manifest, etc.), but it's only effective on the element in the Project Tree that's currently locking availability (see Availability Explained).
The next level down is any Subtotals in the Element. If you click the cogwheel for a Subtotal, then Edit Subtotal, you'll see additional Start and End fields. They will default to the date in the Element header, but if a User clicks in those boxes, they are written as "alternative dates" and will not be overwritten when the Element header is updated. They can be reset by clearing the dates using the paintbrush icons.
The most granular setting level is the line item. By default, these values can be seen on the Warehouse View of a Quote by way of the Pickup Date and Return Date columns. These dates and time are just as sensitive to edits as the Subtotal; a simple click into and out of them will write the alternative date/time and cannot be overwritten by the header. Again, the solution is to click the date, then use the paintbrush icon.