When Does "13 Going on 30" (2004) Take Place?
"13 Going on 30" (released to Australian audiences as "Suddenly 30") is a romantic-comedy fantasy film written by Cathy Yuspa with Josh Goldsmith and Niels Mueller that released April 23rd, 2004. So when is "13 Going on 30" set?
It takes place in the year:
We know this because Jenna Rink's school photo packet is clearly labeled "1987 Tamarack 1987" in the prologue and she’s holding her 13th birthday party (we can also see a height-by-age measuring poster with "12 years old 1986!" written on it hanging on the back of the closet door in Jenna's basement) when Jenna wishes under a sprinkling of magic wishing dust to become "30 and flirty and thriving!" just before the plot really gets going at the 13-minute mark.

2004 AD
We know this because Jenna Rink's school photo packet is clearly labeled "1987 Tamarack 1987" in the prologue and she’s holding her 13th birthday party (we can also see a height-by-age measuring poster with "12 years old 1986!" written on it hanging on the back of the closet door in Jenna's basement) when Jenna wishes under a sprinkling of magic wishing dust to become "30 and flirty and thriving!" just before the plot really gets going at the 13-minute mark.
1986 + 1 = 1987
and
30 - 13 = 17
and
1987 + 17 = 2004.
Additionally, the prologue shows us Jenna on the day before and the day of her 13th birthday party in detail. The audience sees video footage as it's taken on this day and its digital timestamp specifically reads "MAY 26 1987" in the bottom-left corner so we know it's May 26th, 1987 AD.