Automated backup Date variable

Hi,

I am running Fortigate 3000D with fortiOS  6.4.9.

while running the automated backup script

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Config Global

execute backup config tftp  My_FGT_%%date%%.conf 10.3.10.2

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it does not replaces the variable %%date%%  with the date, i even tried the variable %%log.date%% also but no luck.

Can anyone , please help to get the right variable for date.

Regards,

Ali