In Dynamics GP, Period 0 is a special period. It holds the fiscal year beginning balance.
Normally you wouldn't specify it in either the start period or the end period of a Jet GL() function. Period zero balances are automatically included if you omit start period.
Dynamics GP processes period zero balances differently for Open Years versus Historical Years and for Balance Sheet Accounts versus Income Statement Accounts. Jet Reports automatically deals with this complexity if you omit the start period when you want to include the beginning balance. Only one period zero balance is ever included in a returned value, but determining which period zero balance to include can be complex, and this is best left to Jet Reports.
If you specify a period zero in start period, you override Jet Reports. Jet Reports only includes the period zero you specify in the start period, even if this might not result in a valid beginning balance. For example, period zero in all open years except the first one does not include balances for any prior open year.
If you specify period zero in the end period, Jet Reports will only include the balance if the start period is also period zero (or if the start period is omitted and the period zero specified in the end period is the same as the one Jet Reports would normally include.) Jet Reports does not include the balance from a period zero specified in the end period in any other situation.
It may be necessary to specify period zero in the rare case where you have deleted balances for a historical year and need to return a beginning balance as of the first fiscal year with balances. To retrieve the beginning balance, use period zero as both the start period and end period. For example, if you deleted balances for fiscal year 2018 and you wanted to know the beginning balances as of the start of fiscal year 2019, you should put 2019/0 in both start period and end period.