Invalid TMVL return value in Logic Script

Hi Experts,

I stumbled upon this error when executing logic script:

UJK_VALIDATION_EXCEPTION:Invalid TMVL return value

Here is the logic script I created just for testing purpose:

*WHEN TIME

*IS TMVL(13, 2016.12)

///DO Something

*ENDWHEN

However, when I tweak the parameter to:

*WHEN TIME

*IS TMVL(12, 2016.12)

///DO Something

*ENDWHEN

I got no error. I immediately think it was issue from my master data. But then when I check, there seems to be nothing weird with TIME master data.

Please help, particularly if someone can explain how tmvl works. I'm using BPC 10 NW

Thanks,

Hendry