Why would somebody give you money to buy him cryptos?Are you talking about portfolio creation?
Somebody gives you 100$ and you buy 20% btc,25% ethereum,30% monero and 25% litecoin.
I`m pretty sure that a smart contract can do this,but i can`t create smart contracts.
You would need some exchange platform`s API,i guess.
Your idea about <time1> and <time2> which are non-negotiable can`t be executed,because btc transactions are very slow sometimes.