How can I sum strings?

Hi, guys. In order to retrieve a METAR message from JSON file, I need to generate an URL with date in format
Where USER_KEY is my personal key, and INITIAL_DATE and FINAL_DATE should be year + month + day + current hour + 3 hours. Example: now, here where I live, is 14:42 02/09/2021, so I should send 2021020917 (17 because it's 14 + 3 = 17).
I'm using wg to retrieve this JSON and I can do it if I manually input the INITIAL_DATE and FINAL_DATE values, but can't make it get system time and date and create a URL.
I tried $wg("$"" + (df(yyyyMMddh) + 3) + "&data_fim=" + (df(yyyyMMddh) + 3)$", json,[0].mens)$ but it generates an error (err: a literal can't follow another literal).
If i just use $"" + (df(yyyyMMddh) + 3) + "&data_fim=" + (df(yyyyMMddh) + 3)$ I see exactly the URL I need.
Any suggestion?

Try use df("yyyyMMddH", a3h)

H = hour in 0-23 hrs format.
a3h = add 3 hour from now.