"You will need to enter the dates when you order the LOI on our website. The dates you enter when you apply are a guideline only. The LOI when issued is generally valid for 2-3 months and allows you to transit through Turkmenistan within it's validity for up to 5 days.
If it's a land border you are getting into Turkmenistan at, you can get a visa on arrival with the transit LOI and the visa will be valid for 5 days from the date of entry. But if it's the Caspian sea you will be crossing, you'll need to get the visa before boarding the ferry and the final dates will need to be stated when you apply for the actual visa at the embassy either in Baku or in Istanbul"
This answer was directly taken from the Visa Machine's Mongol Rally help pages where they answer loads of your questions.