Networks have billing cycles, where they do "end of month" billing for contract customers.
The "end of month" doesn't necessarily mean the last day in the calendar month, and if for example, they want to process the millions of calls, SMSes, MMSes, mobile internet/broadband billing records for all customers before the end of the calendar month, they'll have to start many days earlier.
Cycles are generally *not* based on when you took out your contract.
From the information you have posted, it appears that you probably got the contract on (or "for billing purposes", it started on) the 18th of February, and that your number happens to have a billing run/cycle that ends on the 23rd of each month.
Therefore they've charged you:
1) in arrears for your use up to the end of the last billing cycle (to February 23rd)
2) In advance for your billing cycle (24th February - 23rd March)
Your next bill will therefore likely be for 24th March to 23rd April.
All utility companies (gas, electric, landline telephone) charge like this