[NOTE: Not applicable to users of HistoryGeo.com for Libraries]
When creating or editing a Custom Marker, you have the option of filling in date-related information that expounds upon the information in the marker's description.
To enter a date or date range into a marker's content, click the "Date Info" button in the "Other (optional)" section near the bottom of the add/edit marker window.
This will open a window where you have the option of entering a date, or a date range.
You have the option of entering just a year, a year and a month, or a full date. Use the check boxes to enable or disable the entry of months or days. Also, if you're unsure about the date, you can click the check box near the bottom of the window to allow you to approximate how far off you think you are.
If you enter a date range, however, you don't have the option of entering approximations. You do, however, still have the option to disable the month and day fields for both the "From" date and the "To" date.
When you're done, click the "OK" button. Once the window has closed you should see that the date or date range you entered will be displayed in the date info box:
If you want to edit the date, just click the "Date Info" button again. To get rid of the date information that you've entered, just click the "Clear" button.