How can I sum strings?

General
1
Hi, guys. In order to retrieve a METAR message from JSON file, I need to generate an URL with date in format https://api-redemet.decea.mil.br/mensagens/metar/SBBI?api_key=USER_KEY&data_ini=INITIAL_DATE&data_fim=FINAL_DATE
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 $wg("$"https://api-redemet.decea.mil.br/mensagens/metar/SBBI?api_key=wv3DfuPYWJOYkf9w1J8EtY8A2tt7bweGGOnBX4mb&data_ini=" + (df(yyyyMMddh) + 3) + "&data_fim=" + (df(yyyyMMddh) + 3)$", json, .data.data[0].mens)$ but it generates an error (err: a literal can't follow another literal).
If i just use $"https://api-redemet.decea.mil.br/mensagens/metar/SBBI?api_key=wv3DfuPYWJOYkf9w1J8EtY8A2tt7bweGGOnBX4mb&data_ini=" + (df(yyyyMMddh) + 3) + "&data_fim=" + (df(yyyyMMddh) + 3)$ I see exactly the URL I need.
Any suggestion?
2

Try use df("yyyyMMddH", a3h)

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