Timing
Contribution Timing: Start vs End of Period (Why It Matters)
Calculators need an assumption about when you contribute. “Start vs end” can slightly change timelines and modeled growth. Here’s what it means and how to test it.
Published: December 22, 2025 · Updated: December 22, 2025 · By FinToolSuite Editorial
Open the calculator
Test start vs end timing with the same goal inputs.
Open the Savings Goal Timeline CalculatorQuick answer
Start-of-period: you add money at the beginning of each week or month.
End-of-period: you add at the end.
Earlier contributions usually produce slightly higher modeled balances when a rate is used.
Month-only timing: start vs end of month.
Disclaimer
Educational purposes only; not financial advice. Examples are illustrative; outcomes aren’t guaranteed. Rates, fees, taxes, and inflation vary by country and provider.
Definitions
Start of period: money is added at the beginning of the week/month (like an annuity due, without the jargon).
End of period: money is added at the end of the week/month (ordinary annuity style).
Many tools default to one assumption; check the setting or help text.
Why results change
- Earlier money has more time to sit in the balance.
- If a rate is used, that extra time can slightly increase modeled growth.
- At 0%, timing usually matters less, though milestones can still shift in some displays.
Illustrative example
Goal: £1,200. Contribution: £100/month. Rate assumption: 3% annual (illustrative) and also consider 0%.
Scenario A: contribute at start of month. Scenario B: contribute at end of month. Scenario A usually ends slightly higher or reaches the goal a bit earlier in the model. The difference is typically small for short timelines and larger over longer ones.
Try both timing options: Savings Goal Timeline Calculator.
Text timeline (illustrative): [Start] £100 → grow → £100 → grow … vs [End] grow → £100 → grow → £100 …
How to test timing
- Enter the same goal, starting balance, contribution amount, and frequency.
- Run with end-of-period assumption.
- Run with start-of-period assumption (if available).
- Keep the rate assumption the same; try 0% first for a baseline.
If the tool doesn’t have a timing toggle, treat results as using the default. For sensitivity, you can approximate earlier timing by adding a small extra contribution in the first period of a test scenario.
When timing matters most
- Longer horizons.
- Higher assumed rates.
- Higher contribution frequency (weekly vs monthly).
- Comparisons keep timing consistent across scenarios.
Common mistakes
- Comparing outputs from two tools with different timing assumptions.
- Mixing “paid on payday” with an end-of-month model without adjusting timing.
- Assuming the timing difference is guaranteed in real life.
FAQ
What does “start of period” mean?
Adding contributions at the beginning of each week or month.
What does “end of period” mean?
Adding contributions at the end of each week or month.
Which one is more realistic?
It depends on your pay and contribution habits. Keep timing consistent when comparing.
Does timing matter if the rate is 0%?
It matters less; the main impact is when milestones appear on the timeline.
Does weekly timing matter more than monthly?
More frequent contributions mean timing differences can add up slightly more.
Is this the same as annuity due vs ordinary annuity?
Conceptually yes, but you can think of it simply as “start vs end.”
Are results guaranteed?
No. Outputs depend on inputs and assumptions.
Where can I learn the formula?
Run two timing scenarios
Use identical inputs, change only timing, and compare the timelines.
Open the Savings Goal Timeline Calculator