The RMD is the amount you must take for the 2018 tax year... you cannot prorate it.
The 2018 amount must be taken in full by 4/1/19 ... you can take some in 2018 and the balance in 2019 but the entire amount must be withdrawn from the IRA account. You don't have to wait until July to start taking the distributions and if you do wait to take the 2018 distributions in 2019 remember that the 2019 distribution MUST be made in 2019. The option to wait is ONLY allowed for the first RMD.
The RMD is the amount you must take for the 2018 tax year... you cannot prorate it.
The 2018 amount must be taken in full by 4/1/19 ... you can take some in 2018 and the balance in 2019 but the entire amount must be withdrawn from the IRA account. You don't have to wait until July to start taking the distributions and if you do wait to take the 2018 distributions in 2019 remember that the 2019 distribution MUST be made in 2019. The option to wait is ONLY allowed for the first RMD.
Just be aware that if you choose to contribute any of this to charity as a Qualified Charitable Distribution, you must wait until you have actually reached age 70½ in July to make the QCD. Any distribution that you receive in 2018 prior to reaching age 70½ will apply toward your 2018 RMD but will not be a QCD.
And I don't think you can delay an RMD from a 401K like you can an IRA which is the info I had in my original answer.
Once the 401K administrator says you must start distributions it must happen ... talk to them about any options you have available to you especially the possibility to roll the entire amount to an IRA (after the current year RMD is satisfied of course).
With regard to my comment about QCDs, I failed to notice that this was in regard to an RMD from a 401(k). You are correct, a QCD cannot be made from a 401(k).
The Required Beginning Date for RMDs from 401(k)s is the same as for IRAs unless the plan agreement specifies otherwise (and I don't know of any reason a plan would specify otherwise). Because 401(k) plans are required to enforce the distribution of RMDs, the 401(k) plan would require the 2018 RMD to be distributed by April 1, 2019.
If you desire to make QCDs in the future, you can roll money from the 401(k) to an IRA and make QCDs from the IRA. However, the 401(k)'sRMD for the year must be satisfied before rolling any amounts over to an IRA. If a rollover to an IRA is done in 2019, the 2019 RMD will have to be satisfied first in addition to any amount of the 2018 RMD that was not yet satisfied.